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Testing 40 predictions from the transtheoretical model
Addictive Behaviors, Vol. 24, No. 4, pp. 455–469, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0306-4603/99/$–see front matter
Pergamon
PII S0306-4603(98)00100-2
TESTING 40 PREDICTIONS FROM THE TRANSTHEORETICAL MODEL WAYNE F. VELICER, GREGORY J. NORMAN, JOSEPH L. FAVA, and JAMES O. PROCHASKA Cancer Prevention Research Center, University of Rhode Island
Abstract — Sensitive measurement of behavior change requires dependent measures that are sensitive to the whole spectrum of change, not just a single aspect of change. Traditional outcome variables such as point prevalence for smoking cessation focus on a single discrete event and ignore all other progress. Alternatively, the criterion measurement model (CMM) is an approach that posits a three-construct outcome model (habit strength, positive evaluation strength, and negative evaluation strength), where different constructs are sensitive to change for different aspects of the temporal domain. In this article, a series of 40 differential a priori predictions were tested using a large representative sample of smokers. The focus was on the prediction of specific effect sizes rather than statistical significance. A series of comparisons involving stage transitions was examined using five variables representing the three CMM constructs. The predictions involved movement from one of three initial stages (precontemplation, contemplation, and preparation) to stage membership 12 months later. Thirtysix of the 40 predictions were confirmed, indicating that the outcome model has strong construct validity and accurately reflects movement between the stages of change. © 1999 Elsevier Science Ltd
Key Words. Stage of change, Decisional Balance, Smoking cessation, Effect size predictions.
Our ability to understand health behavior change and to evaluate related interventions depends greatly on the sensitivity of our outcome measures. Health behavioral scientists are directing more attention toward dynamic process models of change rather than simple static outcome models (Glanz, Lewis, & Rimer, 1997; Miller, 1996). Measuring the complex spectrum of behavior change requires dependent measures that not only assess traditional outcomes but also intermediate outcomes that represent meaningful transitions toward those target behaviors. In this article, a set of process-oriented outcomes is tested in a series of 40 a priori predictions from a theory-based, multivariate outcome model of smoking cessation using a large representative sample of smokers. Velicer, Rossi, Prochaska, and DiClemente (1996) recently proposed a criterion measurement model (CMM) incorporating constructs from the transtheoretical model (TTM) of behavior change (Prochaska & DiClemente, 1983; Prochaska, DiClemente, & Norcross, 1992). The CMM was developed within the context of smoking behavior to measure the multivariate change process that occurs as an individual progresses from being a smoker to becoming a nonsmoker. Point prevalence is the commonly employed outcome measure used by the field. Although it may initially seem to have face validity, Velicer et al. (1996) argued that a traditional outcome measure such as point prevalence suffers from problems of poor definition, poor statistical power, and Grants CA 50087, CA 63045, and CA 27821 from the National Cancer Institute supported this research. An earlier version of this article was presented at the 4th International Congress of Behavioral Medicine, Washington, DC, March 1996. The Web site for the CPRC is WWW.URI.EDU/RESEARCH/CPRC. Requests for reprints should be sent to Wayne F. Velicer, Cancer Prevention Research Center, 2 Chafee Road, University of Rhode Island, Kingston, RI 02881-0808; E-mail: [email protected] 455
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a lack of meaningfulness for sensitive measurement across the complete behavioral change process. As an alternative to traditional problematic outcome variables, a three-construct model was proposed incorporating the constructs of habit strength (the behavioral aspects of smoking), positive evaluation strength (the importance of positive values and beliefs about smoking), and negative evaluation strength (the importance of negative values and beliefs about smoking). Velicer et al. (1996) presented four empirical tests to provide supporting evidence of the structure of the CMM. These tests used a variety of design and analysis procedures including cross-sectional confirmatory factor analysis modeling, graphical representations of a cross-sequential dynamic typology, time series analysis, and longitudinal latent variable panel design modeling. The demonstrated advantages of the CMM over traditional outcome variables are that it consists of constructs rather than single variables, it is multivariate rather than univariate, its measures are continuous rather than discrete, and it assesses longitudinal change rather than only cross-sectional change. The TTM, as a comprehensive theory of behavior change, describes individual movement through a series of five stages, starting with precontemplation, where no behavior change is contemplated, through maintenance, where long-term change has been achieved and is being integrated within the individual’s behavior set. The change process described by the TTM is explicit and allows a complex set of predictions to be made and tested concerning the expected cognitive and behavioral changes that individuals make as they move through the stages of change. These predictions can also be quantified to allow for an extremely rigorous test of the TTM. Unfortunately, traditional outcome measures lack the psychometric sensitivity to assess adequately these characteristic stage-based changes in cognition and behavior. However, the CMM constructs, with their noted psychometric advantages, are ideal for such theory testing. The purpose of the present study was to examine movement through the stages of change using the CMM constructs as dependent measures. Specifically, a series of 40 a priori predictions were made within the first three stages of change and tested in a representative sample of smokers. The predictions will all involve explicit quantitative predictions of the expected effect sizes rather than the weaker directional predictions associated with traditional hypothesis testing approaches.
S T A G E S
O F
C H A N G E
The TTM postulates that behavior change is not a binary phenomenon (Prochaska & DiClemente, 1983). Instead, behavior change is conceptualized as involving movement through a series of discrete stages. Each stage is defined by intentions and behaviors related to the problem behavior of interest. Five stages are included in the TTM. Precontemplation is characterized by resistance to recognizing and modifying a problem behavior. Precontemplators have no intention of changing their behavior in the next 6 months. Contemplators are individuals who are seriously considering changing in the next 6 months. They recognize the problem and know what they want to do, but they are just not ready to act. Both intentions to take action in the near future (next 30 days) and small behavioral changes characterize the preparation stage. The action stage is where behavior is being performed at the criterion level, but this change has taken place within the last 6 months. The maintenance stage represents continuous long-term change but is not a static stage. Rather, in maintenance, the indi-
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vidual works to consolidate behavioral and cognitive/experiential gains made while transitioning through the previous stages and to avoid relapse. P O S I T I V E A N D N E G A T I V E E V A L U A T I O N S T R E N G T H
Positive and negative evaluation strength are two constructs that measure the cognitive changes related to making a change in behavior. Velicer et al. (1996) found that the items of the Decisional Balance Inventory represent excellent measures of these constructs. However, similar constructs are included in most of the major theories of health behavior change. Velicer, DiClemente, Prochaska, and Brandenburg (1985) developed the decisional balance measure for smoking cessation based on Janis and Mann’s (1977) model of decision theory. The original model hypothesized four positive and four negative aspects to consider when making a decision. These included utilitarian gains and losses to self, gains and losses to others, and approval and disapproval from self and important others. Velicer et al. (1985) found that the structure of the decision to change smoking behavior consisted of only two constructs—the pros (positive images, values, and beliefs) and the cons (negative images, values, and beliefs) of change. This basic two-construct model has been replicated for the decisional balance measure across a wide variety of problem behaviors (Prochaska et al., 1994). Figure 1 illustrates the pattern of change for the pros and cons across the stages of change for smoking cessation. The pros are high and the cons are low for precontemplators. As an individual progresses through the early stages of change, the cons of smoking begin to increase to the level of the pros. In the contemplation stage, there is a crossover where the cons become equal to the pros. The cons are higher than the pros in the action stage, but both become less important as individuals move from action to maintenance. The characteristic crossover effect, occurring in either preparation or contemplation, has been identified in 12 different health-related behaviors (Prochaska et al., 1994). H A B I T
S T R E N G T H
Habit strength refers to the psychological and physiological aspects of smoking behavior. One of the best measures of this construct is the Situational Temptations Inventory (Velicer, DiClemente, Rossi, & Prochaska, 1990), which assesses temptations to smoke in three domains: positive/social, negative/affective, and habit/addictive situations. The Situational Temptations Inventory has a hierarchical structure, with three correlated first-order factors representing the three domains and a single second-order factor explaining the high intercorrelations among the first-order scales. This construct is also measured by behavioral variables. For example, in smoking, the average number of cigarettes smoked a day and the time until first cigarette of the day are important indicators of habit strength. However, the Situational Temptations Inventory has the advantage of being an appropriate measure across all the stages of change, whereas some of the behavioral measures are only appropriate for some of the stages. As an individual progresses through the stages of change, habit strength is initially high in precontemplation, decreasing slightly through contemplation and preparation, then decreasing more dramatically through action and into the maintenance stage.
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Fig. 1. Pattern of change for the Decisional Balance scales across the stages of change for smoking cessation.
Habit strength is particularly important for assessing risk for relapse. Individuals who have quit smoking but still report being highly tempted to smoke are at a high risk for relapse. It has been hypothesized that the Habit/Addictive scale will be the first to decrease, followed by the Positive/Social, and then the Negative/Affective (Velicer et al., 1990). Figure 2 illustrates the hypothesized pattern of change for the three scales over the five stages of change.
Fig. 2. Pattern of change for the Situational Temptation Subscales across the stages of change for smoking cessation.
Stage transitions
S T A G E
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T R A N S I T I O N S
By conceptualizing change with the stages of change construct, multiple meaningful outcomes can be assessed. These outcomes are four critical stage transitions: (a) precontemplation to contemplation, (b) contemplation to preparation, (c) preparation to action, and (c) action to maintenance. The traditional outcome measure, point prevalence, is captured by the preparation-to-action transition. Prolonged abstinence is represented by the action-to-maintenance transition. Typically, interventions focus on the preparation-to-action transition and assess outcome using the target behavior. Studies of relapse prevention focus on the action-to-maintenance transition. Little if any attention has been given to the first two early stage transitions, and unfortunately this is where the majority of smokers find themselves (Fava, Velicer, & Prochaska, 1995; Velicer et al., 1995). It is hypothesized that the CMM can provide dependent measures that are sensitive to these critical early stage transitions. Forward-only progress through the stages of change is generally not the typical pattern of change for an individual trying to quit smoking. More often an individual will progress forward for a time and later recycle back to an earlier stage. The recycling process may occur several times before an individual is finally free from smoking. Given an initial starting stage, an individual can be classified as progressing, stable, or regressing, depending upon which stage he or she is classified in on a future assessment. For example, a smoker initially in the contemplation stage who is assessed 6 months later may have made one of three transitions. He or she may have progressed to the preparation or action stage, indicating readiness to quit in the near future (preparation) or that the quitting process has begun (action). He or she may remain stable (i.e., remain in contemplation), continuing to think about quitting but not in the immediate future. A third possibility is to regress back to precontemplation, indicating an increased resistance to quitting smoking. Similar stage transitions can be observed in precontemplation and preparation stage smokers. However, it is not possible to regress from precontemplation because it is the initial stage of change. Figure 3 illustrates progress, stability, and regression for each of the three initial stages. The TTM posits that these transitions will be associated with differences in habit strength, positive evaluation strength, and negative evaluation strength. The present study was designed to test these predictions, using a representative sample of smokers who were followed up 1 year after an initial baseline assessment.
D E V E L O P I N G
Q U A N T I T A T I V E
P R E D I C T I O N S
This article reports the results of three separate prediction studies. Smokers at the baseline assessment were classified into one of the first three stages of change: precontemplation, contemplation, or preparation. These three baseline stage groupings serve to define the samples used in the three studies. Separate analyses of variance were performed using stage transition status 12 months later as the independent grouping variable. The two Decisional Balance subscales (Pros and Cons) and the three Situational Temptations subscales (Positive/Social, Negative/Affective, and Habit/Addictive) served as the dependent variables. A series of a priori predictions was made for each study, where the goal was to predict the effect size for each of a series of orthogonal planned comparisons (Winer, 1971) on each of the dependent variables. The use of orthogonal comparisons ensures that the tests are not partially or completely redundant.
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Fig. 3. Progressing, stable, and regressing stage transitions for individuals starting in the precontemplation, contemplation, and preparation stages.
An emphasis was placed on the magnitude of the effect size rather than solely on statistically significant differences. This represents a shift from the reliance on traditional significance tests (Cohen, 1994). However, significance tests are reported, and the alpha level for all significance tests was set at p , .01. All effect sizes were calculated as omega-squared (v2), the population estimate of the accounted-for variance. Effect size interpretations were based on Cohen’s (1988) descriptive guidelines. A small effect is about 1% of the variance, a medium effect is about 6% of the variance, and a large effect is about 14% or more of the variance. With this approach, predicting an effect size of zero represents a clear prediction. Frick (1996) made an important distinction between ordinal claims and quantitative claims in his discussion about when null hypothesis testing is appropriate in psychological research. Ordinal claims do not specify the size of the effect but only the order or direction of the effect. Alternatively, quantitative claims specify the size of the effect. Frick pointed out that quantitative claims are difficult to make in psychology because effect-size differences often do not generalize well across different populations. As a result, knowledge in psychology is mainly based on ordinal claims and we may have to be satisfied with psychology as an ordinal science. In this case, based on well-developed measures, an explicit theory, and a representative sample, quantitative predic-
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tions were the focus of this article. This is not to say that ordinal claims are not also important but that quantitative claims can be realistic goals in many domains of psychology. F O R T Y
P R E D I C T I O N S
B A S E D
O N
T H E
T T M
Within each study, a set of stage transition comparisons was defined. Table 1 presents the different comparisons for each study including the sample size of each comparison group. For all comparisons, the same five dependent variables were used: pros of smoking, cons of smoking, and the three situational temptations scales. Across the three studies, 40 predictions were made, generated from the eight comparisons on the five dependent variables. For each prediction, the effect size is derived from the degree of change as predicted by the CMM theory and illustrated by the curves in Figures 1 and 2. In Study 1, three sets of transitions, all beginning from precontemplation at baseline, were compared. Comparison 1 was between precontemplators who remained in precontemplation at the 12-month follow-up and precontemplators who progressed to contemplation or preparation (i.e., between smokers who remained stable and smokers who progressed). It was predicted that large differences would only be found between the two groups on the Cons scale. No other outcome measures were expected to differentiate the two groups. Comparison 2 was between precontemplators who progressed one stage to contemplation and precontemplators who progressed two stages to preparation at 12 months. A small difference was expected on the TemptationsHabit/Addictive scale, which is the first temptation expected to change (see Figure 2). No differences were expected on the other two Temptation scales or on the two Decisional Balance scales. Comparison 3 was between all precontemplators who were still smoking at 12 months and all precontemplators who quit smoking at 12 months (i.e., between smokers and nonsmokers). Large differences were expected on the Temptation scales, and moderate differences were predicted for the Pros and Cons. Comparison 1 and Comparison 2 are embedded within Comparison 3. Comparison 1 takes place within the set of individuals still smoking at 12 months. Comparison 2 takes place within the set of individuals still smoking who had progressed at 12 months. A fourth orthogonal comparison, which would have compared precontemplators who reached action with those who reached maintenance, could not be performed because of inadequate sample size. In Study 2, three sets of transitions beginning from contemplation were compared. The first comparison was between contemplators who regressed at 12 months back to precontemplation and contemplators who remained in contemplation (i.e., between smokers who regressed and smokers who remained stable). Large differences were only expected on the Cons scale. In Comparison 2, stable contemplators were compared to contemplators who progressed at least one stage at 12 months. Large differences were expected on the three Temptation scales, moderate differences were expected on the Pros scale, and a small difference was expected on the Cons scale. Comparison 3 was between contemplators who progressed one stage to preparation and contemplators who progressed to action or maintenance. Comparison 3 is embedded within the progressing group in Comparison 2. It distinguishes between contemplators who have made progress but are still smoking and those who have quit smoking. Large differences were predicted for the three Temptation scales and the Pros scale, and a moderate difference was expected on the Cons scale. A fourth orthogonal
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Table 1. Stage transition comparisons and cell sizes for the three within-stage studies Study/comparison Study 1: Precontemplation Comparison 1: PC to PC (smoking, stable; n 5 783) vs. PC to C or PR (smoking, progressing; n 5 309)
Dependent variable
Prediction/effect size
Pros Cons Positive/Social Habit/Addictive Negative/Affective
None Large None None None
Comparison 2: PC to C (smoking, progress one stage; n 5 234) vs. PC to PR (smoking, progress two stages; n 5 75)
Pros Cons Positive/Social Habit/Addictive Negative/Affective
None None None Small None
Comparison 3: PC to PC or C or PR (smoking; n 5 1,092) vs. PC to A or M (nonsmoking; n 5 123)
Pros Cons Positive/Social Habit/Addictive Negative/Affective
Moderate Moderate Large Large Large
Pros Cons Positive/Social Habit/Addictive Negative/Affective
None Large None None None
Comparison 2: C to C (smoking, stable; n 5 535) vs. C to PR or A or M (progressing; n 5 387)
Pros Cons Positive/Social Habit/Addictive Negative/Affective
Moderate Small Large Large Large
Comparison 3: C to PR (smoking, progressing; n 5 169) vs. C to A or M (nonsmoking, progressing; n 5 218)
Pros Cons Positive/Social Habit/Addictive Negative/Affective
Large Moderate Large Large Large
Pros Cons Positive/Social Habit/Addictive Negative/Affective
None Moderate None None None
Pros Cons Positive/Social Habit/Addictive Negative/Affective
Large Large Large Large Large
Study 2: Contemplation Comparison 1: C to PC (smoking, regressing; n 5 295) vs. C to C (smoking, stable; n 5 535)
Study 3: Preparation Comparison 1: PR to C or PC (smoking, regressing; n 5 256) vs. PR to PR (smoking, stable; n 5 153)
Comparison 2: PR to PR (smoking, stable; n 5 153) vs. PR to A or M (nonsmoking; n 5 119)
Note. PC 5 precontemplation; C 5 contemplation; PR 5 preparation; A 5 action; M 5 maintenance.
comparison, which would have compared contemplators who reached action with those who reached maintenance, could not be performed because of inadequate sample size. Two sets of transition comparisons beginning in the preparation stage were examined in Study 3. The first comparison was between those in preparation who regressed back to precontemplation or contemplation at 12 months and those who remained in
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preparation at 12 months. Only moderate differences on the Cons scale were predicted, and no differences were expected on the other four scales. Comparison 2 was between those who remained in preparation and individuals who progressed to the action or maintenance stage. Large differences were expected on the Temptation scales, and on the Pros and Cons scales. A third orthogonal comparison, which would have compared smokers in preparation who regressed to contemplation with those who regressed to precontemplation, could not be performed because of inadequate sample size. A fourth orthogonal comparison, which would have compared smokers in preparation who reached action with those who reached maintenance, also could not be performed because of inadequate sample size. The preparation group was much smaller than the precontemplation and contemplation groups, and the proportion of smokers reaching either of the two extreme groups in 12 months was very small.
M E T H O D
Participants A sample of 4,144 smokers between the ages of 18 and 75 years of age in the State of Rhode Island was recruited by telephone using a random digit dialing method (Lepkowski, 1988; Waksberg, 1978). The mean age of the sample was 41 years, 56% were female, 54% were married, and 95% were White. The median education level was Grade 12. Proactive recruiting of the sample resulted in a distribution that was 42% in precontemplation, 40% in contemplation, and 18% in preparation. Similar proportions of smokers have been found in other representative samples of smokers (Velicer et al., 1995). The participants were part of a large prospective smoking cessation intervention study (Fava et al., 1995). Complete details about the recruitment and sampling procedure can be found in Fava et al. (1995). Information on intervention outcomes from this study can be found in Prochaska, Velicer, Fava, Rossi, and Tsoh (1997). At the 12-month assessment, 28% of the sample were lost to follow-up, creating a cohort with complete data of 2,967 participants. Of these individuals, 461 (15.5%) had quit smoking, placing them in the action or maintenance stage. Of the cohort still smoking, 1,173 (39.5%) were in precontemplation, 936 (31.5%) were in contemplation, and 397 (13.4%) were in the preparation stage at 12-month follow-up. Measures Stages of change. A six-question algorithm was used to classify smokers. At baseline, all participants entered the study as smokers and could only be grouped into one of the first three stages of change: precontemplation, contemplation, or preparation. Decisional balance. The Decisional Balance inventory consists of two constructs that underlie cognitive and motivational aspects of human decision making (Prochaska et al., 1994; Velicer et al., 1985). These constructs have been labeled the “Pros of Smoking” and the “Cons of Smoking,” and they were each measured with three items. Participants responded to each item on a 5-point Likert scale (1 5 not important to 5 5 extremely important). Situational temptations. The Situational Temptations Inventory consists of nine items measuring the temptation to smoke in various situations (Velicer et al., 1990). The nine items represent three subscales labeled Positive/Social, Negative/Affective,
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and Habit/Addictive. Smokers responded to each item on a 5-point Likert scale (1 5 not at all tempted to 5 5 extremely tempted). R E S U L T S
Study 1: Precontemplation Table 2 shows the means, standard deviations, F values, and effect-size estimates for each transition comparison across the five dependent variables. Comparison 1. The progressing group had a Cons score of nearly half a standard deviation higher than the stable precontemplators. This result was in the correct direction but below the predicted magnitude. As predicted no other significant differences were found for the other four variables. Comparison 2. All five predictions were confirmed. As predicted, precontemplators who progressed two stages were higher on the Temptation Habit Strength scale, and the two groups did not differ on the Pros and Cons scales. There were also no differences on the two remaining Temptation scales. Comparison 3. Four of the five predictions were confirmed for this comparison. Precontemplators who progressed to action or maintenance were more than a standard deviation lower on all three Temptation scales compared to precontemplators Table 2. Comparison results for the precontemplation stage Variable Comparison 1 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective Comparison 2 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective Comparison 3 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective
Comparison
ANOVA F value
v2
Prediction confirmed
.04
Yes No
(PC to PC)
(PC to C, PR)
51.1 (11.2) 47.0 (10.1)
51.2 (10.8) 51.6 (10.6)
ns 43.2
52.3 (10.8) 52.7 (10.6) 49.6 (11.1) (PC to C)
52.5 (10.6) 52.1 (10.4) 49.7 (10.3) (PC to PR)
ns ns ns
Yes Yes Yes
51.8 (11.0) 51.2 (10.7)
49.2 (10.1) 52.7 (10.2)
ns ns
Yes Yes
53.2 (10.7) 53.0 (10.3) 50.3 (10.3) (PC to PC, C, PR)
50.4 (10.3) 49.2 (10.2) 48.0 (11.2) (PC to A, M)
51.1 (11.1) 48.3 (10.4)
41.9 (8.6) 46.2 (12.0)
79.8 ns
.06
Yes No
52.4 (10.7) 52.5 (10.6) 49.7 (10.8)
34.6 (13.0) 37.3 (8.5) 33.7 (11.2)
291.2 236.4 236.4
.20 .16 .18
Yes Yes Yes
ns 7.45 ns
.02
Yes Yes Yes
Note. All comparisons used standardized T scores (M 5 50, SD 5 10). Standard deviations appear in parentheses alongside mean values. Degrees of freedom for Comparisons 1, 2, and 3 were (1, 1,015), (1, 303), and (1, 1,213), respectively. Displayed F values were significant at p , .01. ANOVA 5 analysis of variance; PC 5 precontemplation; C 5 contemplation; PR 5 preparation; A 5 action; M 5 maintenance.
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Table 3. Comparison results for the contemplation stage Variable Comparison 1 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective Comparison 2 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective Comparison 3 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective
Comparison
ANOVA F value
v2
Prediction confirmed
.03
Yes No
(C to PC)
(C to C)
51.9 (10.9) 51.0 (10.4)
52.0 (10.2) 54.9 (10.3)
ns 26.9
52.2 (10.5) 51.7 (9.9) 50.7 (10.0) (C to C)
52.4 (10.6) 52.3 (9.6) 50.5 (9.9) (C to PR, A, M)
ns ns ns
52.0 (10.2) 54.9 (10.3)
45.5 (10.6) 50.9 (13.1)
90.4 27.0
.09 .03
Yes Yes
52.4 (10.6) 52.3 (9.6) 50.5 (9.9) (C to PR)
40.7 (14.0) 43.3 (10.5) 40.7 (13.4) (C to A, M)
212.0 181.7 162.0
.19 .16 .15
Yes Yes Yes
49.8 (10.0) 54.8 (11.0)
42.1 (9.8) 47.9 (13.9)
56.7 28.1
.13 .07
Yes Yes
49.9 (11.2) 50.5 (9.2) 48.2 (10.6)
33.5 (11.5) 37.7 (7.7) 34.9 (12.5)
197.0 223.3 121.6
.34 .36 .24
Yes Yes Yes
Yes Yes Yes
Note. All comparisons used standardized T scores (M 5 50, SD 5 10). Standard deviations appear in parentheses alongside mean values. Degrees of freedom for Comparisons 1, 2 and 3 were (1, 828), (1, 270), and (1, 1,385), respectively. Displayed F values were significant at p , .01. ANOVA 5 analysis of variance; C 5 contemplation; PC 5 precontemplation; PR 5 preparation; A 5 action; M 5 motivation.
who were still smoking. Those who had quit smoking were also a standard deviation lower on the pros of smoking compared to the other group. No difference was found for the Cons scale. Study 2: Contemplation Table 3 shows the means, standard deviations, F values, and effect-size estimates for each transition comparison across the five dependent variables. Comparison 1. The group of stable contemplators had a Cons score of nearly half a standard deviation higher than the regressing group. This finding was in the correct direction but below the predicted magnitude. As predicted, no other significant differences were found for the other four variables. Comparison 2. All five predictions were confirmed for this comparison. A moderate effect sizes were found for the Pros scale and a small effect size for the Cons scales, with the progressing group having lower Pros and lower Cons than the stable contemplators. The three Temptation scales were all a standard deviation lower for the progressing group. Comparison 3. Again, all five predictions were confirmed. A moderate to large effect was found for the Pros scale and a moderate effect was found for the Cons scale, with those who progressed to action or maintenance having lower Cons and lower
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Pros than those who progressed to preparation. Differences on the Temptation scales were all well over a standard deviation and in the expected direction. That is, those who had quit smoking were much less tempted to smoke than those who had made progress but were still smoking. Study 3: Preparation Table 4 shows the means, standard deviations, F values, and effect-size estimates for each transition comparison across the five dependent variables. Comparison 1. The stable preparation group had a Cons score nearly half a standard deviation higher than the regressing group. This finding was in the correct direction but below the predicted magnitude. As predicted, no other significant differences were found for the other four variables. Comparison 2. All five predictions were confirmed. Large effect sizes were found for the Pros and Cons scales, with those who progressed to action or maintenance having lower Pros and lower Cons than the stable preparation group. Differences on the Temptation scales were all well over a standard deviation and in the expected direction. That is, those who quit smoking were much less tempted to smoke than those who remained in the preparation stage. D I S C U S S I O N
Forty predictions were made based on construct relationships hypothesized within the TTM. Thirty-six of these predictions were confirmed and four were not. It is important to note that a predication was not considered confirmed, even if the difference was statistically significant, if the magnitude of the difference was not at predicted.
Table 4. Comparison results for the preparation stage Variable Comparison 1 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective Comparison 2 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective
Comparison
ANOVA F value
v2
Prediction confirmed
.03
Yes No
(PC to PC, C)
(PR to PR)
51.1 (10.6) 53.1 (10.6)
51.0 (11.3) 57.1 (9.0)
50.5 (11.2) 51.2 (10.7) 50.6 (10.5) (PR to PR)
51.5 (10.9) 51.6 (9.6) 50.4 (9.3) (PR to A, M)
ns ns ns
51.0 (11.3) 57.1 (9.0)
42.1 (8.4) 48.9 (13.1)
51.8 37.6
.16 .12
Yes Yes
51.5 (10.9) 51.6 (9.6) 50.4 (9.3)
35.0 (11.9) 37.7 (8.3) 37.5 (12.5)
141.6 155.6 95.3
.35 .36 .26
Yes Yes Yes
ns 15.44
Yes Yes Yes
Note. All comparisons used standardized T scores (M 5 50, SD 5 10). Standard deviations appear in parentheses alongside mean values. Degrees of freedom for Comparisons 1 and 2 were (1, 407) and (1, 270), respectively. Displayed F values were significant at p , .01. ANOVA 5 analysis of variance; PR 5 preparation; PC 5 precontemplation; C 5 contemplation; A 5 action; M 5 maintenance.
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Study 1: Precontemplation The three transition comparisons made within the precontemplation stage examined changes in smokers at the baseline assessment who were not considering quitting in the next 6 months. This is a very important group of smokers because they are typically hard to reach, resistant to cessation interventions, and little research has been conducted specifically on these individuals. Thirteen of the 15 precontemplation predictions were confirmed. They indicated that as precontemplators move forward through the early stages, the Cons scores increase from precontemplation to contemplation. This difference was not as large as predicted but was in the expected direction. The pros of smoking remained at the same level through the early stages as predicted. A small difference was expected on the Temptations-Habit/Addictive scale for precontemplators who moved one stage compared to precontemplators who moved two stages to preparation. This prediction was confirmed. It indicates that smokers who progressed two stages seemed to be making changes in their smoking in terms of it being an automatic process (e.g., not smoking when first waking up in the morning). The third comparison, between precontemplators who were still smoking and precontemplators who had quit at 12 months, revealed large differences on all three Situational Temptation scales, indicating that reducing temptation to smoke is closely linked to quitting smoking. As expected, precontemplators who quit smoking at 12 months had much lower pros of smoking compared to those who were still smoking. However, the expected difference on the Cons scale was not found. This finding may be due to the curvilinear nature of the Cons scores across the stages of change. That is, the cons increase from precontemplation to contemplation and then level off and decrease after the action stage. The importance of the negative aspects of smoking is hypothesized to be less important as a person remains free from smoking. Thus, although it is believed that important changes have taken place related to the Cons scores, this analysis does not reflect those changes. However, this comparison may not have been the most direct test because it involves a combination of groups.
Study 2: Contemplation The contemplation stage is a less stable stage than precontemplation. Smokers in this stage are in conflict about their behavior; they want to change, but they do not feel ready to do so in the near future. They could transition back to precontemplation, move forward to preparation or action, or they could remain in contemplation, always planning to change at some point in the more distant future. Fourteen of the 15 predictions concerning this stage were confirmed. The first comparison between stable and regressing contemplators once again indicated that only differences on the Cons scale differentiate precontemplative from contemplative smokers. This difference was in the correct direction and statistically significant but was not as large as predicted. The second comparison between the stable and the progressing contemplators revealed differences on all five variables that matched predicted differences. Those who progressed from contemplation had lower Pros and Cons of Smoking scores and differences in Temptation scores of about a standard deviation compared to those who remained in contemplation. As predicted, the third comparison between contemplators who progressed to preparation and those who quit smoking revealed a similar, yet magnified pattern of results on the criterion outcome measures. These patterns of results revealed that these measures differentially and effectively detect changes as individuals move between stages of change.
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Study 3: Preparation The preparation stage is highly unstable and is that pivotal period when a smoker feels ready to quit, has made a recent quit attempt, and has most likely set a new quit date within the next month. At this point the individual can move forward, taking action by quitting smoking, or regress back to an earlier stage for reasons such as uncertainty or overwhelming life stresses. Nine of the 10 predictions for this stage were confirmed. Once again, for the first comparison, only the Cons of Smoking scores were expected to distinguish those who remained in preparation from those who regressed back to an earlier stage. The magnitude of this difference was not as large as predicted even though the direction was as predicted. The second comparison between those who remained in preparation and those who quit smoking revealed the expected pattern of lower Pros, Cons, and Temptations for those who progressed to the action and maintenance stages. Effect sizes for these differences were very large, explaining up to 36% of the variance.
Applications for intervention From these within-stage sets of comparisons, a number of conclusions can be drawn that can direct and enhance smoking cessation interventions. First, raising the cons of smoking is crucial to forward stage movement from precontemplation to contemplation or preparation. Not increasing the cons increases the likelihood of stage regression among smokers in contemplation and preparation. Second, the pros of smoking do not begin to decrease until the transition from contemplation to preparation. This suggests that targeting the positive aspects of smoking has little impact on early-stage smokers. Changes in the pros of smoking become important during the transition from preparation to action. Third, situational temptations are strongly linked to quitting smoking. Reduced temptations are an important indicator that an individual is ready to quit. Individuals who have recently quit and have strong temptations to smoke are at increased risk of relapse. Identifying and preparing for situations in which an individual is tempted to smoke is an important relapse prevention strategy.
Cautions Maintenance-stage smokers were not differentiated from those in the action stage. The number of smokers who had made it to the maintenance stage after 1 year was not large enough to make meaningful comparisons. Stage transitions from the maintenance stage need to be studied further to determine how the criterion outcome measures relate to risk of relapse and sustained cessation. An important aspect of this study was the use of a representative sample of smokers in a prospective study design. It should be noted that although this was a representative sample, it was not a naturalistic sample. The representative sample was involved in an intervention that generated differential positive effects in both the treatment and the proactive assessment groups (Prochaska et al., 1997). As a result, the changes that occurred over the 12 months coincided with an intervention rather than changes that might occur naturally over time. Although this was a representative sample of smokers, it was from a somewhat homogeneous New England region. Further studies will be needed to determine if these quantitative claims are robust enough to generalize nationally and internationally.
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Construct validity Construct validity (Cronbach & Meehl, 1955) is concerned with the theoretical relationship of a variable with other variables. Confirming 36 of 40 a priori predictions about the criterion outcome measures in relation to the stages of change provides strong evidence of their construct validity. We have successfully predicted when these intermediate outcome measures will be related to stage transitions and when they will not. In addition, in most cases we have successfully predicted the magnitude of these differences. The current study has demonstrated that with proper instrument development and a well-specified theoretical framework, quantitative claims can be hypothesized and tested. These findings elaborate and define the nomological network around the criterion outcome measures. R E F E R E N C E S Cohen, J. (1988). Statistical power analysis for the behavioral sciences. Hillsdale, NJ: Erlbaum. Cohen, J. (1994). The earth is round (p , .05). American Psychologist, 49, 997–1003. Cronbach, L. J., & Meehl, P. E. (1955). Construct validity in psychological tests. Psychological Bulletin, 52, 281–302. Fava, J. L., Velicer, W. F., & Prochaska, J. O. (1995). Applying the transtheoretical model to a representative sample of smokers. Addictive Behaviors, 20, 189–203. Frick, R. W. (1996). The appropriate use of null hypothesis testing. Psychological Methods, 1, 379–390. Glanz, K., Lewis, F. M., & Rimer, B. K. (1997). Linking theory, research, and practice. In K. Glanz, F. M. Lewis, & B. K. Rimer (Eds.), Health behavior and health education: Theory, research, and practice (pp. 19–35). San Francisco: Jossey-Bass. Janis, I. L., & Mann, L. (1977). Decision making: A psychological analysis of conflict, choice and commitment. New York: Free Press. Lepkowski, J. M. (1988). Telephone sampling methods in the United States. In R. M. Groves, R. M. Biemer, L. E. Lyberg, J. T. Massey, W. L. Nicholls II, & J. Waksberg (Eds.), Telephone survey methodology (pp. 73–98). New York: Wiley. Miller, P. M. (1996). Redefining success in eating disorders. Addictive Behaviors, 21, 745–754. Prochaska, J. O., & DiClemente, C. C. (1983). Stages and processes of self-change of smoking: Toward an integrative model of change. Journal of Consulting and Clinical Psychology, 51, 390–395. Prochaska, J. O., DiClemente, C. C., & Norcross, J. C. (1992). In search of how people change: Applications to addictive behaviors. American Psychologist, 47, 1102–1114. Prochaska, J. O., Velicer, W. F., Fava, J. L. Rossi, J. S., & Tsoh, J. Y. (1997). A matched expert system intervention for a total population of smokers. Manuscript under review. Prochaska, J. O., Velicer, W. F., Rossi, J. S., Goldstein, M. G., Marcus, B. H., Rakowski, W., Fiore, C., Harlow, L. L., Redding, C. A., Rosenbloom, D., & Rossi, S. R. (1994). Stages of change and decisional balance for twelve problem behaviors. Health Psychology, 13, 39–46. Velicer, W. F., DiClemente, C. C., Prochaska, J. O., & Brandenburg, N. (1985). Decisional balance measure for assessing and predicting smoking status. Journal of Personality and Social Psychology, 48, 1279– 1289. Velicer, W. F., DiClemente, C. C., Rossi, J. S., & Prochaska, J. O. (1990). Relapse situations and self-efficacy: An integrative model. Addictive Behaviors, 15, 271–283. Velicer, W. F., Fava, J. L., Prochaska, J. O., Abrams, D. B., Emmons, K. M., & Pierce, J. P. (1995). Distribution of smokers by stage in three representative samples. Preventive Medicine, 24, 401–411. Velicer, W. F., Rossi, J. S., Prochaska, J. O., & DiClemente, C. C. (1996). A criterion measurement model for health behavior change. Addictive Behaviors, 21, 555–584. Waksberg, J. (1978). Sampling methods for random digit dialing. Journal of the American Statistical Association, 73, 40–46. Winer, B. J. (1971). Statistical principles in experimental design (2nd ed.). New York: McGraw-Hill.
Pergamon
PII S0306-4603(98)00100-2
TESTING 40 PREDICTIONS FROM THE TRANSTHEORETICAL MODEL WAYNE F. VELICER, GREGORY J. NORMAN, JOSEPH L. FAVA, and JAMES O. PROCHASKA Cancer Prevention Research Center, University of Rhode Island
Abstract — Sensitive measurement of behavior change requires dependent measures that are sensitive to the whole spectrum of change, not just a single aspect of change. Traditional outcome variables such as point prevalence for smoking cessation focus on a single discrete event and ignore all other progress. Alternatively, the criterion measurement model (CMM) is an approach that posits a three-construct outcome model (habit strength, positive evaluation strength, and negative evaluation strength), where different constructs are sensitive to change for different aspects of the temporal domain. In this article, a series of 40 differential a priori predictions were tested using a large representative sample of smokers. The focus was on the prediction of specific effect sizes rather than statistical significance. A series of comparisons involving stage transitions was examined using five variables representing the three CMM constructs. The predictions involved movement from one of three initial stages (precontemplation, contemplation, and preparation) to stage membership 12 months later. Thirtysix of the 40 predictions were confirmed, indicating that the outcome model has strong construct validity and accurately reflects movement between the stages of change. © 1999 Elsevier Science Ltd
Key Words. Stage of change, Decisional Balance, Smoking cessation, Effect size predictions.
Our ability to understand health behavior change and to evaluate related interventions depends greatly on the sensitivity of our outcome measures. Health behavioral scientists are directing more attention toward dynamic process models of change rather than simple static outcome models (Glanz, Lewis, & Rimer, 1997; Miller, 1996). Measuring the complex spectrum of behavior change requires dependent measures that not only assess traditional outcomes but also intermediate outcomes that represent meaningful transitions toward those target behaviors. In this article, a set of process-oriented outcomes is tested in a series of 40 a priori predictions from a theory-based, multivariate outcome model of smoking cessation using a large representative sample of smokers. Velicer, Rossi, Prochaska, and DiClemente (1996) recently proposed a criterion measurement model (CMM) incorporating constructs from the transtheoretical model (TTM) of behavior change (Prochaska & DiClemente, 1983; Prochaska, DiClemente, & Norcross, 1992). The CMM was developed within the context of smoking behavior to measure the multivariate change process that occurs as an individual progresses from being a smoker to becoming a nonsmoker. Point prevalence is the commonly employed outcome measure used by the field. Although it may initially seem to have face validity, Velicer et al. (1996) argued that a traditional outcome measure such as point prevalence suffers from problems of poor definition, poor statistical power, and Grants CA 50087, CA 63045, and CA 27821 from the National Cancer Institute supported this research. An earlier version of this article was presented at the 4th International Congress of Behavioral Medicine, Washington, DC, March 1996. The Web site for the CPRC is WWW.URI.EDU/RESEARCH/CPRC. Requests for reprints should be sent to Wayne F. Velicer, Cancer Prevention Research Center, 2 Chafee Road, University of Rhode Island, Kingston, RI 02881-0808; E-mail: [email protected] 455
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a lack of meaningfulness for sensitive measurement across the complete behavioral change process. As an alternative to traditional problematic outcome variables, a three-construct model was proposed incorporating the constructs of habit strength (the behavioral aspects of smoking), positive evaluation strength (the importance of positive values and beliefs about smoking), and negative evaluation strength (the importance of negative values and beliefs about smoking). Velicer et al. (1996) presented four empirical tests to provide supporting evidence of the structure of the CMM. These tests used a variety of design and analysis procedures including cross-sectional confirmatory factor analysis modeling, graphical representations of a cross-sequential dynamic typology, time series analysis, and longitudinal latent variable panel design modeling. The demonstrated advantages of the CMM over traditional outcome variables are that it consists of constructs rather than single variables, it is multivariate rather than univariate, its measures are continuous rather than discrete, and it assesses longitudinal change rather than only cross-sectional change. The TTM, as a comprehensive theory of behavior change, describes individual movement through a series of five stages, starting with precontemplation, where no behavior change is contemplated, through maintenance, where long-term change has been achieved and is being integrated within the individual’s behavior set. The change process described by the TTM is explicit and allows a complex set of predictions to be made and tested concerning the expected cognitive and behavioral changes that individuals make as they move through the stages of change. These predictions can also be quantified to allow for an extremely rigorous test of the TTM. Unfortunately, traditional outcome measures lack the psychometric sensitivity to assess adequately these characteristic stage-based changes in cognition and behavior. However, the CMM constructs, with their noted psychometric advantages, are ideal for such theory testing. The purpose of the present study was to examine movement through the stages of change using the CMM constructs as dependent measures. Specifically, a series of 40 a priori predictions were made within the first three stages of change and tested in a representative sample of smokers. The predictions will all involve explicit quantitative predictions of the expected effect sizes rather than the weaker directional predictions associated with traditional hypothesis testing approaches.
S T A G E S
O F
C H A N G E
The TTM postulates that behavior change is not a binary phenomenon (Prochaska & DiClemente, 1983). Instead, behavior change is conceptualized as involving movement through a series of discrete stages. Each stage is defined by intentions and behaviors related to the problem behavior of interest. Five stages are included in the TTM. Precontemplation is characterized by resistance to recognizing and modifying a problem behavior. Precontemplators have no intention of changing their behavior in the next 6 months. Contemplators are individuals who are seriously considering changing in the next 6 months. They recognize the problem and know what they want to do, but they are just not ready to act. Both intentions to take action in the near future (next 30 days) and small behavioral changes characterize the preparation stage. The action stage is where behavior is being performed at the criterion level, but this change has taken place within the last 6 months. The maintenance stage represents continuous long-term change but is not a static stage. Rather, in maintenance, the indi-
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vidual works to consolidate behavioral and cognitive/experiential gains made while transitioning through the previous stages and to avoid relapse. P O S I T I V E A N D N E G A T I V E E V A L U A T I O N S T R E N G T H
Positive and negative evaluation strength are two constructs that measure the cognitive changes related to making a change in behavior. Velicer et al. (1996) found that the items of the Decisional Balance Inventory represent excellent measures of these constructs. However, similar constructs are included in most of the major theories of health behavior change. Velicer, DiClemente, Prochaska, and Brandenburg (1985) developed the decisional balance measure for smoking cessation based on Janis and Mann’s (1977) model of decision theory. The original model hypothesized four positive and four negative aspects to consider when making a decision. These included utilitarian gains and losses to self, gains and losses to others, and approval and disapproval from self and important others. Velicer et al. (1985) found that the structure of the decision to change smoking behavior consisted of only two constructs—the pros (positive images, values, and beliefs) and the cons (negative images, values, and beliefs) of change. This basic two-construct model has been replicated for the decisional balance measure across a wide variety of problem behaviors (Prochaska et al., 1994). Figure 1 illustrates the pattern of change for the pros and cons across the stages of change for smoking cessation. The pros are high and the cons are low for precontemplators. As an individual progresses through the early stages of change, the cons of smoking begin to increase to the level of the pros. In the contemplation stage, there is a crossover where the cons become equal to the pros. The cons are higher than the pros in the action stage, but both become less important as individuals move from action to maintenance. The characteristic crossover effect, occurring in either preparation or contemplation, has been identified in 12 different health-related behaviors (Prochaska et al., 1994). H A B I T
S T R E N G T H
Habit strength refers to the psychological and physiological aspects of smoking behavior. One of the best measures of this construct is the Situational Temptations Inventory (Velicer, DiClemente, Rossi, & Prochaska, 1990), which assesses temptations to smoke in three domains: positive/social, negative/affective, and habit/addictive situations. The Situational Temptations Inventory has a hierarchical structure, with three correlated first-order factors representing the three domains and a single second-order factor explaining the high intercorrelations among the first-order scales. This construct is also measured by behavioral variables. For example, in smoking, the average number of cigarettes smoked a day and the time until first cigarette of the day are important indicators of habit strength. However, the Situational Temptations Inventory has the advantage of being an appropriate measure across all the stages of change, whereas some of the behavioral measures are only appropriate for some of the stages. As an individual progresses through the stages of change, habit strength is initially high in precontemplation, decreasing slightly through contemplation and preparation, then decreasing more dramatically through action and into the maintenance stage.
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Fig. 1. Pattern of change for the Decisional Balance scales across the stages of change for smoking cessation.
Habit strength is particularly important for assessing risk for relapse. Individuals who have quit smoking but still report being highly tempted to smoke are at a high risk for relapse. It has been hypothesized that the Habit/Addictive scale will be the first to decrease, followed by the Positive/Social, and then the Negative/Affective (Velicer et al., 1990). Figure 2 illustrates the hypothesized pattern of change for the three scales over the five stages of change.
Fig. 2. Pattern of change for the Situational Temptation Subscales across the stages of change for smoking cessation.
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T R A N S I T I O N S
By conceptualizing change with the stages of change construct, multiple meaningful outcomes can be assessed. These outcomes are four critical stage transitions: (a) precontemplation to contemplation, (b) contemplation to preparation, (c) preparation to action, and (c) action to maintenance. The traditional outcome measure, point prevalence, is captured by the preparation-to-action transition. Prolonged abstinence is represented by the action-to-maintenance transition. Typically, interventions focus on the preparation-to-action transition and assess outcome using the target behavior. Studies of relapse prevention focus on the action-to-maintenance transition. Little if any attention has been given to the first two early stage transitions, and unfortunately this is where the majority of smokers find themselves (Fava, Velicer, & Prochaska, 1995; Velicer et al., 1995). It is hypothesized that the CMM can provide dependent measures that are sensitive to these critical early stage transitions. Forward-only progress through the stages of change is generally not the typical pattern of change for an individual trying to quit smoking. More often an individual will progress forward for a time and later recycle back to an earlier stage. The recycling process may occur several times before an individual is finally free from smoking. Given an initial starting stage, an individual can be classified as progressing, stable, or regressing, depending upon which stage he or she is classified in on a future assessment. For example, a smoker initially in the contemplation stage who is assessed 6 months later may have made one of three transitions. He or she may have progressed to the preparation or action stage, indicating readiness to quit in the near future (preparation) or that the quitting process has begun (action). He or she may remain stable (i.e., remain in contemplation), continuing to think about quitting but not in the immediate future. A third possibility is to regress back to precontemplation, indicating an increased resistance to quitting smoking. Similar stage transitions can be observed in precontemplation and preparation stage smokers. However, it is not possible to regress from precontemplation because it is the initial stage of change. Figure 3 illustrates progress, stability, and regression for each of the three initial stages. The TTM posits that these transitions will be associated with differences in habit strength, positive evaluation strength, and negative evaluation strength. The present study was designed to test these predictions, using a representative sample of smokers who were followed up 1 year after an initial baseline assessment.
D E V E L O P I N G
Q U A N T I T A T I V E
P R E D I C T I O N S
This article reports the results of three separate prediction studies. Smokers at the baseline assessment were classified into one of the first three stages of change: precontemplation, contemplation, or preparation. These three baseline stage groupings serve to define the samples used in the three studies. Separate analyses of variance were performed using stage transition status 12 months later as the independent grouping variable. The two Decisional Balance subscales (Pros and Cons) and the three Situational Temptations subscales (Positive/Social, Negative/Affective, and Habit/Addictive) served as the dependent variables. A series of a priori predictions was made for each study, where the goal was to predict the effect size for each of a series of orthogonal planned comparisons (Winer, 1971) on each of the dependent variables. The use of orthogonal comparisons ensures that the tests are not partially or completely redundant.
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Fig. 3. Progressing, stable, and regressing stage transitions for individuals starting in the precontemplation, contemplation, and preparation stages.
An emphasis was placed on the magnitude of the effect size rather than solely on statistically significant differences. This represents a shift from the reliance on traditional significance tests (Cohen, 1994). However, significance tests are reported, and the alpha level for all significance tests was set at p , .01. All effect sizes were calculated as omega-squared (v2), the population estimate of the accounted-for variance. Effect size interpretations were based on Cohen’s (1988) descriptive guidelines. A small effect is about 1% of the variance, a medium effect is about 6% of the variance, and a large effect is about 14% or more of the variance. With this approach, predicting an effect size of zero represents a clear prediction. Frick (1996) made an important distinction between ordinal claims and quantitative claims in his discussion about when null hypothesis testing is appropriate in psychological research. Ordinal claims do not specify the size of the effect but only the order or direction of the effect. Alternatively, quantitative claims specify the size of the effect. Frick pointed out that quantitative claims are difficult to make in psychology because effect-size differences often do not generalize well across different populations. As a result, knowledge in psychology is mainly based on ordinal claims and we may have to be satisfied with psychology as an ordinal science. In this case, based on well-developed measures, an explicit theory, and a representative sample, quantitative predic-
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tions were the focus of this article. This is not to say that ordinal claims are not also important but that quantitative claims can be realistic goals in many domains of psychology. F O R T Y
P R E D I C T I O N S
B A S E D
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T H E
T T M
Within each study, a set of stage transition comparisons was defined. Table 1 presents the different comparisons for each study including the sample size of each comparison group. For all comparisons, the same five dependent variables were used: pros of smoking, cons of smoking, and the three situational temptations scales. Across the three studies, 40 predictions were made, generated from the eight comparisons on the five dependent variables. For each prediction, the effect size is derived from the degree of change as predicted by the CMM theory and illustrated by the curves in Figures 1 and 2. In Study 1, three sets of transitions, all beginning from precontemplation at baseline, were compared. Comparison 1 was between precontemplators who remained in precontemplation at the 12-month follow-up and precontemplators who progressed to contemplation or preparation (i.e., between smokers who remained stable and smokers who progressed). It was predicted that large differences would only be found between the two groups on the Cons scale. No other outcome measures were expected to differentiate the two groups. Comparison 2 was between precontemplators who progressed one stage to contemplation and precontemplators who progressed two stages to preparation at 12 months. A small difference was expected on the TemptationsHabit/Addictive scale, which is the first temptation expected to change (see Figure 2). No differences were expected on the other two Temptation scales or on the two Decisional Balance scales. Comparison 3 was between all precontemplators who were still smoking at 12 months and all precontemplators who quit smoking at 12 months (i.e., between smokers and nonsmokers). Large differences were expected on the Temptation scales, and moderate differences were predicted for the Pros and Cons. Comparison 1 and Comparison 2 are embedded within Comparison 3. Comparison 1 takes place within the set of individuals still smoking at 12 months. Comparison 2 takes place within the set of individuals still smoking who had progressed at 12 months. A fourth orthogonal comparison, which would have compared precontemplators who reached action with those who reached maintenance, could not be performed because of inadequate sample size. In Study 2, three sets of transitions beginning from contemplation were compared. The first comparison was between contemplators who regressed at 12 months back to precontemplation and contemplators who remained in contemplation (i.e., between smokers who regressed and smokers who remained stable). Large differences were only expected on the Cons scale. In Comparison 2, stable contemplators were compared to contemplators who progressed at least one stage at 12 months. Large differences were expected on the three Temptation scales, moderate differences were expected on the Pros scale, and a small difference was expected on the Cons scale. Comparison 3 was between contemplators who progressed one stage to preparation and contemplators who progressed to action or maintenance. Comparison 3 is embedded within the progressing group in Comparison 2. It distinguishes between contemplators who have made progress but are still smoking and those who have quit smoking. Large differences were predicted for the three Temptation scales and the Pros scale, and a moderate difference was expected on the Cons scale. A fourth orthogonal
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Table 1. Stage transition comparisons and cell sizes for the three within-stage studies Study/comparison Study 1: Precontemplation Comparison 1: PC to PC (smoking, stable; n 5 783) vs. PC to C or PR (smoking, progressing; n 5 309)
Dependent variable
Prediction/effect size
Pros Cons Positive/Social Habit/Addictive Negative/Affective
None Large None None None
Comparison 2: PC to C (smoking, progress one stage; n 5 234) vs. PC to PR (smoking, progress two stages; n 5 75)
Pros Cons Positive/Social Habit/Addictive Negative/Affective
None None None Small None
Comparison 3: PC to PC or C or PR (smoking; n 5 1,092) vs. PC to A or M (nonsmoking; n 5 123)
Pros Cons Positive/Social Habit/Addictive Negative/Affective
Moderate Moderate Large Large Large
Pros Cons Positive/Social Habit/Addictive Negative/Affective
None Large None None None
Comparison 2: C to C (smoking, stable; n 5 535) vs. C to PR or A or M (progressing; n 5 387)
Pros Cons Positive/Social Habit/Addictive Negative/Affective
Moderate Small Large Large Large
Comparison 3: C to PR (smoking, progressing; n 5 169) vs. C to A or M (nonsmoking, progressing; n 5 218)
Pros Cons Positive/Social Habit/Addictive Negative/Affective
Large Moderate Large Large Large
Pros Cons Positive/Social Habit/Addictive Negative/Affective
None Moderate None None None
Pros Cons Positive/Social Habit/Addictive Negative/Affective
Large Large Large Large Large
Study 2: Contemplation Comparison 1: C to PC (smoking, regressing; n 5 295) vs. C to C (smoking, stable; n 5 535)
Study 3: Preparation Comparison 1: PR to C or PC (smoking, regressing; n 5 256) vs. PR to PR (smoking, stable; n 5 153)
Comparison 2: PR to PR (smoking, stable; n 5 153) vs. PR to A or M (nonsmoking; n 5 119)
Note. PC 5 precontemplation; C 5 contemplation; PR 5 preparation; A 5 action; M 5 maintenance.
comparison, which would have compared contemplators who reached action with those who reached maintenance, could not be performed because of inadequate sample size. Two sets of transition comparisons beginning in the preparation stage were examined in Study 3. The first comparison was between those in preparation who regressed back to precontemplation or contemplation at 12 months and those who remained in
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preparation at 12 months. Only moderate differences on the Cons scale were predicted, and no differences were expected on the other four scales. Comparison 2 was between those who remained in preparation and individuals who progressed to the action or maintenance stage. Large differences were expected on the Temptation scales, and on the Pros and Cons scales. A third orthogonal comparison, which would have compared smokers in preparation who regressed to contemplation with those who regressed to precontemplation, could not be performed because of inadequate sample size. A fourth orthogonal comparison, which would have compared smokers in preparation who reached action with those who reached maintenance, also could not be performed because of inadequate sample size. The preparation group was much smaller than the precontemplation and contemplation groups, and the proportion of smokers reaching either of the two extreme groups in 12 months was very small.
M E T H O D
Participants A sample of 4,144 smokers between the ages of 18 and 75 years of age in the State of Rhode Island was recruited by telephone using a random digit dialing method (Lepkowski, 1988; Waksberg, 1978). The mean age of the sample was 41 years, 56% were female, 54% were married, and 95% were White. The median education level was Grade 12. Proactive recruiting of the sample resulted in a distribution that was 42% in precontemplation, 40% in contemplation, and 18% in preparation. Similar proportions of smokers have been found in other representative samples of smokers (Velicer et al., 1995). The participants were part of a large prospective smoking cessation intervention study (Fava et al., 1995). Complete details about the recruitment and sampling procedure can be found in Fava et al. (1995). Information on intervention outcomes from this study can be found in Prochaska, Velicer, Fava, Rossi, and Tsoh (1997). At the 12-month assessment, 28% of the sample were lost to follow-up, creating a cohort with complete data of 2,967 participants. Of these individuals, 461 (15.5%) had quit smoking, placing them in the action or maintenance stage. Of the cohort still smoking, 1,173 (39.5%) were in precontemplation, 936 (31.5%) were in contemplation, and 397 (13.4%) were in the preparation stage at 12-month follow-up. Measures Stages of change. A six-question algorithm was used to classify smokers. At baseline, all participants entered the study as smokers and could only be grouped into one of the first three stages of change: precontemplation, contemplation, or preparation. Decisional balance. The Decisional Balance inventory consists of two constructs that underlie cognitive and motivational aspects of human decision making (Prochaska et al., 1994; Velicer et al., 1985). These constructs have been labeled the “Pros of Smoking” and the “Cons of Smoking,” and they were each measured with three items. Participants responded to each item on a 5-point Likert scale (1 5 not important to 5 5 extremely important). Situational temptations. The Situational Temptations Inventory consists of nine items measuring the temptation to smoke in various situations (Velicer et al., 1990). The nine items represent three subscales labeled Positive/Social, Negative/Affective,
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and Habit/Addictive. Smokers responded to each item on a 5-point Likert scale (1 5 not at all tempted to 5 5 extremely tempted). R E S U L T S
Study 1: Precontemplation Table 2 shows the means, standard deviations, F values, and effect-size estimates for each transition comparison across the five dependent variables. Comparison 1. The progressing group had a Cons score of nearly half a standard deviation higher than the stable precontemplators. This result was in the correct direction but below the predicted magnitude. As predicted no other significant differences were found for the other four variables. Comparison 2. All five predictions were confirmed. As predicted, precontemplators who progressed two stages were higher on the Temptation Habit Strength scale, and the two groups did not differ on the Pros and Cons scales. There were also no differences on the two remaining Temptation scales. Comparison 3. Four of the five predictions were confirmed for this comparison. Precontemplators who progressed to action or maintenance were more than a standard deviation lower on all three Temptation scales compared to precontemplators Table 2. Comparison results for the precontemplation stage Variable Comparison 1 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective Comparison 2 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective Comparison 3 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective
Comparison
ANOVA F value
v2
Prediction confirmed
.04
Yes No
(PC to PC)
(PC to C, PR)
51.1 (11.2) 47.0 (10.1)
51.2 (10.8) 51.6 (10.6)
ns 43.2
52.3 (10.8) 52.7 (10.6) 49.6 (11.1) (PC to C)
52.5 (10.6) 52.1 (10.4) 49.7 (10.3) (PC to PR)
ns ns ns
Yes Yes Yes
51.8 (11.0) 51.2 (10.7)
49.2 (10.1) 52.7 (10.2)
ns ns
Yes Yes
53.2 (10.7) 53.0 (10.3) 50.3 (10.3) (PC to PC, C, PR)
50.4 (10.3) 49.2 (10.2) 48.0 (11.2) (PC to A, M)
51.1 (11.1) 48.3 (10.4)
41.9 (8.6) 46.2 (12.0)
79.8 ns
.06
Yes No
52.4 (10.7) 52.5 (10.6) 49.7 (10.8)
34.6 (13.0) 37.3 (8.5) 33.7 (11.2)
291.2 236.4 236.4
.20 .16 .18
Yes Yes Yes
ns 7.45 ns
.02
Yes Yes Yes
Note. All comparisons used standardized T scores (M 5 50, SD 5 10). Standard deviations appear in parentheses alongside mean values. Degrees of freedom for Comparisons 1, 2, and 3 were (1, 1,015), (1, 303), and (1, 1,213), respectively. Displayed F values were significant at p , .01. ANOVA 5 analysis of variance; PC 5 precontemplation; C 5 contemplation; PR 5 preparation; A 5 action; M 5 maintenance.
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Table 3. Comparison results for the contemplation stage Variable Comparison 1 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective Comparison 2 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective Comparison 3 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective
Comparison
ANOVA F value
v2
Prediction confirmed
.03
Yes No
(C to PC)
(C to C)
51.9 (10.9) 51.0 (10.4)
52.0 (10.2) 54.9 (10.3)
ns 26.9
52.2 (10.5) 51.7 (9.9) 50.7 (10.0) (C to C)
52.4 (10.6) 52.3 (9.6) 50.5 (9.9) (C to PR, A, M)
ns ns ns
52.0 (10.2) 54.9 (10.3)
45.5 (10.6) 50.9 (13.1)
90.4 27.0
.09 .03
Yes Yes
52.4 (10.6) 52.3 (9.6) 50.5 (9.9) (C to PR)
40.7 (14.0) 43.3 (10.5) 40.7 (13.4) (C to A, M)
212.0 181.7 162.0
.19 .16 .15
Yes Yes Yes
49.8 (10.0) 54.8 (11.0)
42.1 (9.8) 47.9 (13.9)
56.7 28.1
.13 .07
Yes Yes
49.9 (11.2) 50.5 (9.2) 48.2 (10.6)
33.5 (11.5) 37.7 (7.7) 34.9 (12.5)
197.0 223.3 121.6
.34 .36 .24
Yes Yes Yes
Yes Yes Yes
Note. All comparisons used standardized T scores (M 5 50, SD 5 10). Standard deviations appear in parentheses alongside mean values. Degrees of freedom for Comparisons 1, 2 and 3 were (1, 828), (1, 270), and (1, 1,385), respectively. Displayed F values were significant at p , .01. ANOVA 5 analysis of variance; C 5 contemplation; PC 5 precontemplation; PR 5 preparation; A 5 action; M 5 motivation.
who were still smoking. Those who had quit smoking were also a standard deviation lower on the pros of smoking compared to the other group. No difference was found for the Cons scale. Study 2: Contemplation Table 3 shows the means, standard deviations, F values, and effect-size estimates for each transition comparison across the five dependent variables. Comparison 1. The group of stable contemplators had a Cons score of nearly half a standard deviation higher than the regressing group. This finding was in the correct direction but below the predicted magnitude. As predicted, no other significant differences were found for the other four variables. Comparison 2. All five predictions were confirmed for this comparison. A moderate effect sizes were found for the Pros scale and a small effect size for the Cons scales, with the progressing group having lower Pros and lower Cons than the stable contemplators. The three Temptation scales were all a standard deviation lower for the progressing group. Comparison 3. Again, all five predictions were confirmed. A moderate to large effect was found for the Pros scale and a moderate effect was found for the Cons scale, with those who progressed to action or maintenance having lower Cons and lower
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Pros than those who progressed to preparation. Differences on the Temptation scales were all well over a standard deviation and in the expected direction. That is, those who had quit smoking were much less tempted to smoke than those who had made progress but were still smoking. Study 3: Preparation Table 4 shows the means, standard deviations, F values, and effect-size estimates for each transition comparison across the five dependent variables. Comparison 1. The stable preparation group had a Cons score nearly half a standard deviation higher than the regressing group. This finding was in the correct direction but below the predicted magnitude. As predicted, no other significant differences were found for the other four variables. Comparison 2. All five predictions were confirmed. Large effect sizes were found for the Pros and Cons scales, with those who progressed to action or maintenance having lower Pros and lower Cons than the stable preparation group. Differences on the Temptation scales were all well over a standard deviation and in the expected direction. That is, those who quit smoking were much less tempted to smoke than those who remained in the preparation stage. D I S C U S S I O N
Forty predictions were made based on construct relationships hypothesized within the TTM. Thirty-six of these predictions were confirmed and four were not. It is important to note that a predication was not considered confirmed, even if the difference was statistically significant, if the magnitude of the difference was not at predicted.
Table 4. Comparison results for the preparation stage Variable Comparison 1 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective Comparison 2 Decisional Balance Pros Cons Situational Temptation Positive/Social Habit/Addictive Negative/Affective
Comparison
ANOVA F value
v2
Prediction confirmed
.03
Yes No
(PC to PC, C)
(PR to PR)
51.1 (10.6) 53.1 (10.6)
51.0 (11.3) 57.1 (9.0)
50.5 (11.2) 51.2 (10.7) 50.6 (10.5) (PR to PR)
51.5 (10.9) 51.6 (9.6) 50.4 (9.3) (PR to A, M)
ns ns ns
51.0 (11.3) 57.1 (9.0)
42.1 (8.4) 48.9 (13.1)
51.8 37.6
.16 .12
Yes Yes
51.5 (10.9) 51.6 (9.6) 50.4 (9.3)
35.0 (11.9) 37.7 (8.3) 37.5 (12.5)
141.6 155.6 95.3
.35 .36 .26
Yes Yes Yes
ns 15.44
Yes Yes Yes
Note. All comparisons used standardized T scores (M 5 50, SD 5 10). Standard deviations appear in parentheses alongside mean values. Degrees of freedom for Comparisons 1 and 2 were (1, 407) and (1, 270), respectively. Displayed F values were significant at p , .01. ANOVA 5 analysis of variance; PR 5 preparation; PC 5 precontemplation; C 5 contemplation; A 5 action; M 5 maintenance.
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Study 1: Precontemplation The three transition comparisons made within the precontemplation stage examined changes in smokers at the baseline assessment who were not considering quitting in the next 6 months. This is a very important group of smokers because they are typically hard to reach, resistant to cessation interventions, and little research has been conducted specifically on these individuals. Thirteen of the 15 precontemplation predictions were confirmed. They indicated that as precontemplators move forward through the early stages, the Cons scores increase from precontemplation to contemplation. This difference was not as large as predicted but was in the expected direction. The pros of smoking remained at the same level through the early stages as predicted. A small difference was expected on the Temptations-Habit/Addictive scale for precontemplators who moved one stage compared to precontemplators who moved two stages to preparation. This prediction was confirmed. It indicates that smokers who progressed two stages seemed to be making changes in their smoking in terms of it being an automatic process (e.g., not smoking when first waking up in the morning). The third comparison, between precontemplators who were still smoking and precontemplators who had quit at 12 months, revealed large differences on all three Situational Temptation scales, indicating that reducing temptation to smoke is closely linked to quitting smoking. As expected, precontemplators who quit smoking at 12 months had much lower pros of smoking compared to those who were still smoking. However, the expected difference on the Cons scale was not found. This finding may be due to the curvilinear nature of the Cons scores across the stages of change. That is, the cons increase from precontemplation to contemplation and then level off and decrease after the action stage. The importance of the negative aspects of smoking is hypothesized to be less important as a person remains free from smoking. Thus, although it is believed that important changes have taken place related to the Cons scores, this analysis does not reflect those changes. However, this comparison may not have been the most direct test because it involves a combination of groups.
Study 2: Contemplation The contemplation stage is a less stable stage than precontemplation. Smokers in this stage are in conflict about their behavior; they want to change, but they do not feel ready to do so in the near future. They could transition back to precontemplation, move forward to preparation or action, or they could remain in contemplation, always planning to change at some point in the more distant future. Fourteen of the 15 predictions concerning this stage were confirmed. The first comparison between stable and regressing contemplators once again indicated that only differences on the Cons scale differentiate precontemplative from contemplative smokers. This difference was in the correct direction and statistically significant but was not as large as predicted. The second comparison between the stable and the progressing contemplators revealed differences on all five variables that matched predicted differences. Those who progressed from contemplation had lower Pros and Cons of Smoking scores and differences in Temptation scores of about a standard deviation compared to those who remained in contemplation. As predicted, the third comparison between contemplators who progressed to preparation and those who quit smoking revealed a similar, yet magnified pattern of results on the criterion outcome measures. These patterns of results revealed that these measures differentially and effectively detect changes as individuals move between stages of change.
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Study 3: Preparation The preparation stage is highly unstable and is that pivotal period when a smoker feels ready to quit, has made a recent quit attempt, and has most likely set a new quit date within the next month. At this point the individual can move forward, taking action by quitting smoking, or regress back to an earlier stage for reasons such as uncertainty or overwhelming life stresses. Nine of the 10 predictions for this stage were confirmed. Once again, for the first comparison, only the Cons of Smoking scores were expected to distinguish those who remained in preparation from those who regressed back to an earlier stage. The magnitude of this difference was not as large as predicted even though the direction was as predicted. The second comparison between those who remained in preparation and those who quit smoking revealed the expected pattern of lower Pros, Cons, and Temptations for those who progressed to the action and maintenance stages. Effect sizes for these differences were very large, explaining up to 36% of the variance.
Applications for intervention From these within-stage sets of comparisons, a number of conclusions can be drawn that can direct and enhance smoking cessation interventions. First, raising the cons of smoking is crucial to forward stage movement from precontemplation to contemplation or preparation. Not increasing the cons increases the likelihood of stage regression among smokers in contemplation and preparation. Second, the pros of smoking do not begin to decrease until the transition from contemplation to preparation. This suggests that targeting the positive aspects of smoking has little impact on early-stage smokers. Changes in the pros of smoking become important during the transition from preparation to action. Third, situational temptations are strongly linked to quitting smoking. Reduced temptations are an important indicator that an individual is ready to quit. Individuals who have recently quit and have strong temptations to smoke are at increased risk of relapse. Identifying and preparing for situations in which an individual is tempted to smoke is an important relapse prevention strategy.
Cautions Maintenance-stage smokers were not differentiated from those in the action stage. The number of smokers who had made it to the maintenance stage after 1 year was not large enough to make meaningful comparisons. Stage transitions from the maintenance stage need to be studied further to determine how the criterion outcome measures relate to risk of relapse and sustained cessation. An important aspect of this study was the use of a representative sample of smokers in a prospective study design. It should be noted that although this was a representative sample, it was not a naturalistic sample. The representative sample was involved in an intervention that generated differential positive effects in both the treatment and the proactive assessment groups (Prochaska et al., 1997). As a result, the changes that occurred over the 12 months coincided with an intervention rather than changes that might occur naturally over time. Although this was a representative sample of smokers, it was from a somewhat homogeneous New England region. Further studies will be needed to determine if these quantitative claims are robust enough to generalize nationally and internationally.
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Construct validity Construct validity (Cronbach & Meehl, 1955) is concerned with the theoretical relationship of a variable with other variables. Confirming 36 of 40 a priori predictions about the criterion outcome measures in relation to the stages of change provides strong evidence of their construct validity. We have successfully predicted when these intermediate outcome measures will be related to stage transitions and when they will not. In addition, in most cases we have successfully predicted the magnitude of these differences. The current study has demonstrated that with proper instrument development and a well-specified theoretical framework, quantitative claims can be hypothesized and tested. These findings elaborate and define the nomological network around the criterion outcome measures. R E F E R E N C E S Cohen, J. (1988). Statistical power analysis for the behavioral sciences. Hillsdale, NJ: Erlbaum. Cohen, J. (1994). The earth is round (p , .05). American Psychologist, 49, 997–1003. Cronbach, L. J., & Meehl, P. E. (1955). Construct validity in psychological tests. Psychological Bulletin, 52, 281–302. Fava, J. L., Velicer, W. F., & Prochaska, J. O. (1995). Applying the transtheoretical model to a representative sample of smokers. Addictive Behaviors, 20, 189–203. Frick, R. W. (1996). The appropriate use of null hypothesis testing. Psychological Methods, 1, 379–390. Glanz, K., Lewis, F. M., & Rimer, B. K. (1997). Linking theory, research, and practice. In K. Glanz, F. M. Lewis, & B. K. Rimer (Eds.), Health behavior and health education: Theory, research, and practice (pp. 19–35). San Francisco: Jossey-Bass. Janis, I. L., & Mann, L. (1977). Decision making: A psychological analysis of conflict, choice and commitment. New York: Free Press. Lepkowski, J. M. (1988). Telephone sampling methods in the United States. In R. M. Groves, R. M. Biemer, L. E. Lyberg, J. T. Massey, W. L. Nicholls II, & J. Waksberg (Eds.), Telephone survey methodology (pp. 73–98). New York: Wiley. Miller, P. M. (1996). Redefining success in eating disorders. Addictive Behaviors, 21, 745–754. Prochaska, J. O., & DiClemente, C. C. (1983). Stages and processes of self-change of smoking: Toward an integrative model of change. Journal of Consulting and Clinical Psychology, 51, 390–395. Prochaska, J. O., DiClemente, C. C., & Norcross, J. C. (1992). In search of how people change: Applications to addictive behaviors. American Psychologist, 47, 1102–1114. Prochaska, J. O., Velicer, W. F., Fava, J. L. Rossi, J. S., & Tsoh, J. Y. (1997). A matched expert system intervention for a total population of smokers. Manuscript under review. Prochaska, J. O., Velicer, W. F., Rossi, J. S., Goldstein, M. G., Marcus, B. H., Rakowski, W., Fiore, C., Harlow, L. L., Redding, C. A., Rosenbloom, D., & Rossi, S. R. (1994). Stages of change and decisional balance for twelve problem behaviors. Health Psychology, 13, 39–46. Velicer, W. F., DiClemente, C. C., Prochaska, J. O., & Brandenburg, N. (1985). Decisional balance measure for assessing and predicting smoking status. Journal of Personality and Social Psychology, 48, 1279– 1289. Velicer, W. F., DiClemente, C. C., Rossi, J. S., & Prochaska, J. O. (1990). Relapse situations and self-efficacy: An integrative model. Addictive Behaviors, 15, 271–283. Velicer, W. F., Fava, J. L., Prochaska, J. O., Abrams, D. B., Emmons, K. M., & Pierce, J. P. (1995). Distribution of smokers by stage in three representative samples. Preventive Medicine, 24, 401–411. Velicer, W. F., Rossi, J. S., Prochaska, J. O., & DiClemente, C. C. (1996). A criterion measurement model for health behavior change. Addictive Behaviors, 21, 555–584. Waksberg, J. (1978). Sampling methods for random digit dialing. Journal of the American Statistical Association, 73, 40–46. Winer, B. J. (1971). Statistical principles in experimental design (2nd ed.). New York: McGraw-Hill.