Substance use clusters in a college sample: A multitheoretical approach

Journal of Substance Abuse, 6, 45-66 (1994)

Substance Use Clusters in a College Sample: A Multitheoretical Approach P. Allison Minugh Brown University

Lisa L. Harlow University of Rhode Island

Substance use factors were examined and cross-validated in a cluster analytic approach in two independent samples of college students: N = 291 and N -- 602. Reported frequency, intensity, and amount of substance use were examined for beer, hard liquor, marijuana, amphetamines, barbiturates, psychedelics, cocaine, and heroin. Variables were reduced using Principal Components Analysis (PCA) to form four substance use composites. Composite scores were entered into two different methods of cluster analysis that each identified four distinct clusters of substance use groups. External validity was obtained by showing that these four groups differed on a set of relevant variables. T h e four groups served as levels of the independent variable, substance use type, in four MANOVAs examining group differences on peer and family influence, psychosocial functioning, habit acquisition, and self-efficacy. Findings indicate that as frequency and intensity of substance use increased, individuals reported more problems in living, although a causal direction cannot be established. T h e findings are of potential value in early identification, prevention, and education regarding substance use among college populations.

An important question facing our modern postindustrial society concerns which individuals are at high risk of misusingalcohol and other drugs. Attempts to examine the substance use problem often involve a search for global answers suitable for all substance users and all forms of substance abuse. This is difficult because drug use crosses a wide array of demographics such as age, gender, race, religious affiliation, income/social class, ethnic groups, and academic status (e.g., Brennan, Walfish, & AuBuchon, 1986b; Kandel, 1982; Paton & Kandel, 1978). It is further complicated by findings (e.g., Huba, Wingard, & Bender, 1979) suggesting that there is not a single, general drug use subculture in which beginning users remain throughout their drug use experimentation (see also Gorsuch

This project was based on the first author's masters thesis, University of Rhode Island. T h e research was supported in part by a Biomedical Support Grant from N I H to the University of Rhode Island and Grant CA50087 from the National Cancer Institute (Dr. J. O. Prochaska, P.I.) Appreciation is extended to Drs. Alan Willoughby, Wayne Velicer, Art Stein, and Lynn Pasquerella for comments on an earlier draft. Correspondence and requests for reprints should be sent to P. Allison Minugh, Center for Alcohol and Addiction Studies, Box G-BH, Brown University, Providence, RI 02912. 45

46

P.A. Minugh and L.L. Harlow

& Butler, 1976). Thus, the study of substance users as one group of homogeneous individuals may in part account for some of the contradictory findings in the literature. This study investigated whether substance users could be clustered based on their frequency, intensity, and amount of substance use. Additionally, the clusters were explored to determine whether various groups of substance users could be differentiated on a set of relevant characteristics and behaviors. Prior research has identified at least four basic factors that appear to set drug users apart from their nonusing counterparts: social learning from peers and family, psychosocial functioning, stages of habit acquisition, and self-efficacy.

Social Learning Theory Explanation of Substance Use An individual's peer and family influences play a primary role in substance use and misuse. Social learning theory suggests that individuals model their behavior after that of people in the immediate environment (Bandura, 1977b). Substance use, in this framework, is added to the individual's repertoire of behaviors by virtue of the fact that substance use behavior has been observed among the individual's peers or family. Previously modeled substance-using behavior may serve as a guide for future action, provided the opportunity to practice the behavior exists in the individual's environment. Behavioral science research has frequently identified peers as prime models of substance use behavior (Kandel, 1982; Kaplan, Martin, & Robbins, 1984). Peers are seemingly one of the strongest influences on alcohol and drug abuse (Brennan, Walfish, & AuBuchon, 1986a; Kandel, 1982), although the substance user's influence set also encompasses parents who use the same substances themselves (Collins & Marlatt, 1981; Dishion & Loeber, 1985; Hansen et al., 1987; Huba & Bentler, 1982; Huba, Wingard, & Bender, 1979; Needle et al., 1986).

Psychosocial Theory of Substance Use Over the years, a variety of psychosocial variables related to substance use have been identified, including factors such as self-esteem, self-derogation, selfcriticality, and self-acceptance (e.g., Brennan, Walfish, & AuBuchon, 1986a; Harlow, Newcomb, & Bentler, 1986; Kaplan, Martin, & Robbins, 1984). For example, Kaplan et al. (1984) found self-derogation and involvement with drugusing peers to be related to substance use at a later date. Other variables shown to be related to substance use include depressive mood, normlessness, powerlessness, suicidal ideation, and lack of purpose in life (Crumbaugh & Carr, 1979; Harlow, Newcomb, & Benfler, 1986; Newcomb, Bentler, & Collins, 1986; Newcomb & Harlow, 1986; Padelford, 1974; Paton & Kandel, 1978; Shean & Fechtmann, 1971).

Stage Theory of Substance Use A third factor is the notion that the initiation process for drug use (e.g., cocaine, heroin) apparently proceeds in a series of stages. Kandel (1975) found a series of drug use stages among adolescents that proceeded from no use to the

Substance Use Clusters

47

use of beer and wine, then cigarettes and hard liquor, followed by marijuana, and finally other illicit drugs. Following adolescents into adulthood, Kandel, Yamaguchi, and Chen (1992) found continued support for the stage theory. Using as their basis research conducted by Prochaska & DiClemente (1982, 1983), Stern, Prochaska, Velicer, and Elder (1987) investigated another form of stage theory proposing that cigarette smoking habit acquisition occurs in a series of progressive stages. The scale they developed to measure smoking habit acquisition has been adapted for use in identifying stages of cocaine habit acquisition (Harlow & Minugh, 1989). Similar to the findings related to smoking acquisition (Stern et al., 1987; Elder et al., 1990), the cocaine habit acquisition scale identifies three reliable stages of habit acquisition: Precontemplation, for those who do not plan to start using cocaine; Decision Making, describing individuals who have either considered using cocaine or have begun to experiment with it but do not have plans for continued use; and a Maintenance stage for those individuals who regularly use cocaine and plan continued use (Harlow & Minugh, 1989). Examination of substance use as a series of stages does not necessarily imply that a user will move along the continuum to greater substance use at a later stage, although it has been found that individuals reaching later stages of substance use have significantly more experience with substances at earlier stages (Harlow & Minugh, 1989; Huba & Bentler, 1982; Kandel, 1975, 1982; Kaplan, Martin, & Robbins, 1984).

Self-Efficacy Theory of Substance Use Self-efficacy has been identified as another potentially mediating variable related to behavior such as substance use (Bandura, 1977a; Velicer, DiClemente, Rossi, & Prochaska, 1990). Self-efficacy, or the individual's behavioral expectations, is important in the individual's perception of situations in which alcohol and other substances are available. DiClemente (1981) and his colleagues (DiClemente, Prochaska, & Gibertini, 1985) constructed a scale for the measurement of perceived efficacy in situations considered tempting for cigarette smoking. Based on the work of DiClemente et al. (1985), a scale assessing temptations for drinking alcohol in various situations was adopted (Harlow, 1987, 1989). The scale asks how tempted one would be to drink in various social and negative situations and out of habit. It is expected that different substance use types would show differential temptations to drink alcohol in varying situations. Research has found that cocaine users are significantly more likely to drink alcohol in social situations, when distressed and out of habit, than noncocaine users (Harlow & Minugh, 1989). The focus of this study addressed the underlying assumption involved in examining substance use from a single variable perspective. The study presented here incorporates these four theories of substance use in an attempt to assess a theoretically unified set of variables related to substance use, given various types of substance users. Groups were expected to differ on measures designed to tap theories explaining substance use in terms of social modeling, psychosocial variables, stages of use, and self efficacy.

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P.A. Minugh and L.L. Harlow

Hypotheses Using principal component analysis, a large set of substance use variables can be reduced to a few descriptive "substance class" composites that capture much of the variation in the variables. These composites will then serve as levels of an independent variable, substance use type, in subsequent cluster analyses. Cluster analytic methods will identify at least three distinct substance use groups: an alcohol use group and two drug use groups. One of the drug use groups would primarily use marijuana, psychedelics, amphetamines, and barbiturates as well as alcohol. The other drug use group would use cocaine, or crack, and heroin as well as drugs from the other groups. These groups were expected, based on both previous research (Clifford, Edmundson, Koch, & Dodd, 1989; Gorsuch & Butler, 1976; Huba et al., 1979; Paton & Kandel, 1978) and statistics regarding drug and alcohol use patterns among college students (USDHHS, 1987). Testing social learning theory would reveal the following: (a) Peer pressure is significantly associated with the use of alcohol, cannabis, amphetamines, barbiturates, psychedelics, cocaine, crack, and heroin; and (b) a positive alcohol and/or drug history will exist in the families and friends of individuals who use drugs. An examination of psychosocial variables would show that participants who use more illicit drugs would score lower on dependent measures of purpose in life and score higher on self-derogation and suicide ideation. The potential for a lack of strength in this hypothesis is supported by literature indicating that these variables have a weaker association in the presence of other variables (such as peer pressure) often used in substance use research (Kandel, 1982; Kaptan, Martin, & Robbins, 1984; Paton & Kandel, 1978). Analyses testing the stage theory of substance use will find the following: (a) Those who cluster in a group using primarily alcohol will be in a Precontemplation stage regarding desire to use cocaine; (b) those who cluster in a group that uses alcohol as well as other drugs (i.e., marijuana, amphetamines, barbiturates, psychedelics) will tend to be in the Decision Making stage of cocaine habit acquisition; and (c) Those who cluster in the group using cocaine and heroin will tend to be in the Maintenance stage of cocaine habit acquisition. Self-efficacy in temptations to use alcohol will differ across substance use groups such that individuals in groups using more substances with greater intensity will report greater temptation to use alcohol. Cluster analysis results in Study 1 will be replicated in a second independent sample. Support for this hypothesis should verify both the number and content of the substance use clusters. External validity analyses for the clusters, using MANOVAs on four sets of variables, will be replicated in a second independent sample to demonstrate that the substance use clusters can be reliably differentiated.

METHOD Participants Data were previously collected at two independent time points on two different samples of college students at a New England university. In the first study, 291 participants (212 women and 79 men) were sampled; the mean age was 21.6

Substance Use Clusters

49

years. In Study 2, the sample included data from 602 participants; the mean age was 19.9 in this sample, with 401 women and 201 men responding. The first sample was used for the initial calibration, and a random subset of equal size was taken from the second sample and served as the validation sample for all analyses. Because cluster analyses are usually conducted on smaller samples that represent a larger set of subjects (Lorr, 1987), a random subset of 291 participants was selected by the computer owing to the large number of individuals in the second data set (N = 602) and to provide numerical similarity in the two samples. The existence of two data sets made cross-validation possible and added internal validity to findings from the initial analysis. Procedure

A survey was administered to all participants with various measures of substance use and psychosocial variables. The variables used in the cluster analysis focused on the frequency, intensity, and amount consumed of various substances (i.e., beer, wine, hard liquor, marijuana, hashish, amphetamines, barbiturates, psychedelics, crack and other cocaine, and heroin). Frequency was operationally defined as the participants' report of how often they had used any one of the substances in the past 6 months, ranging from no use to using several times on a daily basis. Intensity was the participants' perception of how often they overindulged in any of the specified substances in the past 6 months, ranging from not at all to perceptions of overindulgence on a daily basis. Frequency and intensity were measured using a l-to-5 Likert-type format, with higher numbers indicating more frequent and intense use of the substances in question. Amount was a measure of "how much" of the substance was used on those days when the substances were used in the past 6 months. This was measured using a 1-to-5 Likert-type rating for each substance. For example, amount of cocaine use for the past month was measured on the following scale: (a) no use, (b) less than 1/2 gram, (c) 1/2 to 1 gram, (d) 1 to 2 grams, and (e) 2 grams or more daily. The clusters formed from these variables would serve as levels of an independent variable, substance use type. A second set of measures was used as dependent variables in MANOVA analyses to validate differences among groups identified by the cluster analysis. All of the following measures used a Likert-type scale that ranged from (1) indicating low or no endorsement to either (4) or (5) indicating high endorsement of that item.

Peer Influence. To determine the extent that individuals perceived pressure from their peers, measures were taken from a series of questions designed to tap peer influences (Harlow, 1987). A variable "alcohol pressure" was formed from the average of 3 items assessing the degree of perceived peer pressure to use beer, wine, and hard liquor. Combination scores also were made for peer pressure to use cannabis and other illicit drugs. Substance Use Network. This is a set of 10 questions intended to determine whether participants have a substance using network of family members (parents

50

P.A. Minugh and L.L. Harlow

or siblings), peers, and/or relatives who have experienced problems with alcohol or other drugs (Harlow, 1987). Two composites were formed: The first was the average of 5 items related to individuals in the participant's network who experienced alcohol problems; the second was the average of 5 items concerning those in the network having problems with drugs other than alcohol.

Self-Derogation. The self-derogation scale is a 7-item measure used by Kaplan (1976) in which 5 items inquire into negative self-perceptions and the other 2 items focus on a positive self-image (Kaplan, 1976). The scale has been found to be reliable with alpha coefficient internal consistency ratings of .79 and .83 (Kaplan et al., 1984). Suicide Ideation. The suicide ideation scale (Harlow, Newcomb, & Bentler, 1986) is a 5-item Likert-type measure based on a scale adapted from Zung (1974) with responses ranging from never to always. The scale has been shown to have an internal consistency coefficient of .80 (Harlow, Newcomb, & Bentler, 1987). Purpose in Life. The purpose in life scale (PILS) is a 20-item test that was developed by Crumbaugh and Maholick (1964) and later revised by Harlow, Newcomb, and Bentler (1987). The PILS has a reliability coefficient of .85, corrected by the Spearman-Brown formula to .92, and an internal consistency of .86 (Crumbaugh & Maholick, 1964; Harlow et al., 1987). Cocaine Habit Acquisition Scale. A Cocaine Habit Acquisition Scale (CHAS) was developed by Harlow (1987) based on an adaptation of Stern et al.'s (1987) scale that reliably identified stages of smoking habit acquisition. The CHAS consists of 20 items with three factors found to be reliable (Harlow & Minugh, 1989): Precontemplating cocaine use (cx = .95), Decision Making about cocaine use (cx = .91), and Maintenance of a cocaine habit (cx = .79).

Self-Efficacy. The Temptation for Alcohol Scale, assessing temptations to drink alcohol, was adapted by Harlow (1987) from DiClemente et al.'s (1985) items on temptation to smoke. The scale has three composites focusing on social, distress, and habit temptations that are internally valid and reliable for alcohol with alpha coefficients of .89, .94, and .84 respectively (DiClemente, 1981; DiClemente et al., 1985; Harlow, 1987).

Analyses Four sets of analyses were conducted on the data: (a) a principal component analysis of the substance use items to be included in the cluster analysis; (b) cluster analyses on the component composites; (c) MANOVAs on the substance use groups with four sets of dependent variables; and (d) cross-validation of the Principal Component Analysis (PCA), cluster analyses, and MANOVAs in a second independent sample. An exploratory PCA was conducted in Study 1 on all of the substances (wine,

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51

beer, hard liquor, marijuana, hashish, amphetamines, barbiturates, psychedelics, powdered and crack cocaine, and heroin) in their various forms of reported use (frequency, intensity, and amount). This procedure was used to find relationships among the variables to reduce the set of variables used in the cluster analysis and to select composites that would adequately reflect the participants' reported use. These composites then served as clustering variables in both the initial cluster analysis (Study 1) and the replication study (Study 2). The second set of analyses, involving the cluster analysis procedure, was performed on composite variables formed from the participants' reported frequency, intensity, and amount of substance use for three sets of substances: alcohol, marijuana, and other substances as identified via the PCA (amphetamines, barbiturates, psychedelics, powdered and crack cocaine, and heroin). The cluster analysis program, PROC CLUSTER (SAS Institute, 1985), was used to methodologically investigate the participants' responses to questions based on their reported substance use, using scaled composites. This analysis was conducted using both Ward's and Average linkage methods. The third set of analyses employed the substance use groups formed by the cluster analyses as levels of an independent variable, substance use type in a series of four one-way MANOVAs. These analyses compared the cluster-formed substance use groups on dependent measures of (a) peer and family influence, (b) psychosocial characteristics, (c) stages of habit acquisition, and (d) self-efficacy. According to the four theories being tested, the MANOVAs were expected to locate group differences on the dependent variables. Follow-up ANOVAs were conducted on significant MANOVAs using significance levels controlling for the Type I error rate in the large number of analyses. In the fourth set of analyses (Study 2), the results were replicated using a second independent sample to cross-validate the findings in Study 1. A second PCA was conducted on the data set to validate the formation of the clustering variables. Additionally, the cluster analysis and MANOVAs were replicated using a random subset (N = 291) of participants from the larger data set containing 602 participants. Significant results from both data sets, and from both sets of analyses, would thus lend credibility to a substance use typology showing significant differences across substance use groups on a number of outcome measures.

RESULTS T h e results of this study indicate that individuals in a college sample can be reliably grouped and differentiated on a variety of variables related to substance use according to their chosen substances and patterns of use. Findings were replicated across both studies.

Study 1 Principal Component Analysis An initial exploratory PCA was conducted using SAS, PROC FACTOR (SAS, 1985), on the participants' self-reported frequency, intensity, and amount mea-

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P.A. Minugh and L.L. Harlow

sures for beer, wine, hard liquor, marijuana, hashish, amphetamines, barbiturates, psychedelics, powdered and crack cocaine, and heroin. Based on a scree plot and interpretability, a four-component solution was retained, explaining 64% of the variance in the variables. Both Varimax and Promax rotation methods were utilized and yielded similar solutions. Those variables that were not complex (i.e., did not load highly on more than one factor) and had loadings of .50 or better were retained on the components. Scaled composites were formed from each of the components by averaging across retained variables. This would provide relatively orthogonal composites for use in the cluster analyses that are more stable across samples than actual component scores. The following substance-class composites were formed: alcohol (the average measure of beer and hard liquor frequency, intensity, and amount of use), marijuana (the average frequency, intensity, and amount of marijuana use), drug frequency (the average frequency of amphetamines, barbiturates, psychedelics, cocaine, and heroin), and drug intensity (the average intensity of amphetamines, barbiturates, psychedelics, cocaine, crack, and heroin). Measures concerned with wine use and hashish use did not clearly load on any of the factors and were not retained for use in the final composites. Once the composite variables were formed, they were standardized to t scores with a mean of 50 and a standard deviation of 10 using the SAS PROC STANDARD program. Standardizing the scores ensures that variables with varying metrics are not differentially weighted when forming clusters. These four standardized composite variables were then assessed using cluster analysis to determine whether several "substance use types" would emerge. Cluster Analysis T scores for the four scaled composites were entered into the first of two cluster analyses using Ward's linkage method. Based on an examination of the dendrogram, a clustering tree diagramming the participants, the cluster analysis using the Ward's linkage method clearly revealed four distinct substance use groups that were validated in two ways. First, the four clusters were internally validated by conducting a second cluster analysis using the Average linkage method on the same data. This also yielded four clusters that were comparable to those obtained by Ward's method, although there were some differences in cluster size between the two methods. Second, the clusters were externally validated on a set of variables not used in the clustering procedure as recommended by Aldenderfer and Blashfield (1984). The following clusters were identified: a Low Alcohol Use Group (N = 90); an Alcohol Use Group (N = 114); an Alcohol and Marijuana Use Group (N - 67); and a Multiple Substance Use Group (N 20). To externally validate the substance use groups identified by the cluster analysis in this sample, MANOVAs were conducted to investigate each of the four substance use theories. A graph of the means for these four substance use groups plotted against the clustering variables is presented in Figure 1. In each MANOVA, the independent variable was the substance use group with the previously discussed four levels (low alcohol use, alcohol use, alcohol and marijuana use, and multiple substance use). The dependent variables for

53

Substance Use Clusters

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Figure 1. Mean cluster scores on alcohol, marijuana, drug frequency, and drug intensity for Study 1.

each MANOVA focused on the following f o u r areas: (1) substance use network, (2) psychosocial functioning, (3) cocaine habit acquisition, and (4) self-efficacy, respectively. MANOVA 1: S u b s t a n c e U s e N e t w o r k T h e first MANOVA e x a m i n e d substance use network variables and resulted in significance for Wilks's )~ = .63, accounting for 37% o f the variance, F ( 1 5 , 7 8 2 ) = 9.57, p = .0001. Follow-up ANOVAs on the d e p e n d e n t variables that examined perceived pressure to use alcohol, marijuana, and o t h e r substances were significant. T h e ANOVA that e x a m i n e d pressure to use alcohol was significant, F(3, 287) = 4.47, p < .004; however, there were no significant g r o u p differences. Pressure to use marijuana was significant, F(3, 287) = 44.59, p < .0001. In this case, multiple substance users r e p o r t e d m o r e p e e r pressure to use marijuana t h a n did the o t h e r t h r e e groups. Additionally, the alcohol and marijuana use g r o u p rep o r t e d m o r e pressure to use marijuana than those in the low use and alcohol use groups. Pressure to use drugs o t h e r than alcohol and marijuana also was r e p o r t ed differently according to substance use typology, F(3, 287) = 12.20, p < .0001, with multiple substance users r e p o r t i n g the most pressure to use drugs, followed by the alcohol and marijuana use group, which also d i f f e r e d f r o m the low use

54

P.A. Minugh and L.L. Harlow

Table 1.

Means o f Dependent Variables Measured for Cluster Groups: Study 1

Variable

Low Use

Alcohol Use

Alcohol & Marijuana Use

Multiple Substance Use

p

2.91 1.64 1.12 4.78 5.32

3.42 1.86 1.27 4.62 5.46

3.46 2.55 1.37 3.81 5.12

3.60 3.70 2.07 2.70 3.61

** **** *** * *

1.65 3.93 1.25

1.65 3.87 1.32

1.67 3.88 1.31

2.00 3.64 1.73

n.s. n.s. **

2.34 1.50 1.22

2.91 1.76 1.42

3.20 2.23 1.67

3.57 2.59 2.29

**** **** ****

4.59 1.19 1.05

4.55 1.28 1.06

3.58 2.18 1.47

2.86 3.15 2.33

**** **** ** **

Peer/Family Influence Pressure to Use Alcohol Pressure to Use Marijuana Pressure to Use D r u g s Alcohol Use Network D r u g Use Network

Psychosocial Factors Self-Derogation P u r p o s e in Life Suicide Ideation

Alcohol Temptations I n Social Situations In Negative Situations O u t o f Habit

Cocaine Habit Acquisition Precontemplation Decision-making Maintenance *p < .05;

**p < .01;

***p < .001;

****p < .0001.

and the alcohol use groups. The univariate test examining alcohol use network was significant, F(3, 287) = 2.99, p < .03, as was the ANOVA on drug use network, F(3, 287) = 2.82, p < .04; however, no significant group differences were found in follow-up analyses, although the means indicate that there may be a trend for individuals using more substances with greater frequency to have family histories of substance use. Group means on each of the variables examined in Study 1 for each MANOVA are presented in Table 1.

MANOVA 2: Psychosocial Variables T h e second MANOVA investigated the relationship between substance use groups and participants' measures on the psychosocial variables self-derogation, purpose in life, and suicide ideation. The value for Wilks's k in this analysis was .95, indicating that substance use types explained only 5% of the variance, F(9, 694) = 1.69, p < .09. Although the MANOVA was only marginally significant, univariate analyses were examined to discern any detectable trends. It was found that both self derogation, F(3, 287) = 1.54, p = .21, n.s., and purpose in life were nonsignificant, F(3, 287) = 1.85, p = .14, n.s.; however, the ANOVA examining suicide ideation achieved a significant p value, F(3, 287) = 4.54, p < .004, indicating that individuals in the multiple substance use group reported more morbid thoughts than did others.

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55

MANOVA 3: Cocaine Habit Acquisition The third MANOVA examined three habit stage variables related to intentions to never start (Precontemplation stage), to experiment (Decision Making stage), or to maintain cocaine use (Maintenance stage). The results of this analysis were significant, Wilks's k = .52, accounting for 48% of the variance, F(9,694) = 23.49, p < .0001. The univariate test of the dependent variable Precontemplation was significant, F(3, 287) = 27.84, p < .0001. Respondents in the low use and alcohol use groups reported having no intentions of ever starting cocaine use and were significantly different from individuals in the alcohol and marijuana and multiple substance use groups for whom this was not true. Analysis of the Decision Making stage revealed that individuals differed according to substance use group on this variable, F(3, 287) = 77.76, p < .0001. Participants in the multiple substance use group reported having experimented more with cocaine than individuals in the other three groups. Those in the alcohol and marij u a n a use group also reported more experimentation with cocaine than those in the low use and alcohol use groups. The dependent variable Maintenance stage also was significant, F(3,287) = 34.45, p < .0001. In this case, multiple substance users (in particular) and the alcohol and marijuana users had no intention of quitting their cocaine use. MANOVA 4: Alcohol Temptations The fourth MANOVA examined the participants' reported scores for temptations (i.e., nonself-efficacy) to drink in social and negative situations and out of habit. The overall results were significant, Wilks's k = .63, accounting for 37% of the variance, F(9, 694) = 16.19, p < .0001. Groups differed significantly when asked whether they would be tempted to drink in a variety of social situations, F(3,287) = 34.22, p < .0001. Participants in the low use group reported the least amount of temptations to drink in social situations. Additionally, the multiple substance use group reported being more tempted to drink in social situations than did those in the alcohol use group, even though these individuals reported the heaviest drinking. The second univariate ANOVA revealed a significant effect for temptations to drink in negative situations, F(3, 287) = 19.96,/7 < .0001. Multiple substance users and those in the alcohol and marijuana use group did not differ from each other; however, they reported drinking more often in negative situations than did individuals in both the low use and the alcohol use groups. T h e results of the cluster analysis and these four MANOVAs indicate that individuals can be differentiated according to substance use type, and given their typologies, they can be further differentiated on a variety of dependent measures that have been historically related to substance use. The results of this study were replicated on a second independent sample of N = 291 college students with similar demographics. Study 2

A replication across time and samples of the results reported in Study 1 were found using a randomly selected sample of 291 participants from a second inde-

56

P.A.

Minugh and L.L. Harlow

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Figure 2. Mean cluster scores on alcohol, marijuana, drug frequency, and drug intensity for Study 2.

p e n d e n t data set. A second Principal C o m p o n e n t s Analysis on the replication data set revealed that the same clustering variables should be used: alcohol, marijuana, d r u g frequency, and d r u g intensity. Scores on these variables were stand a r d i z e d to T scores with a m e a n o f 50 and a standard deviation o f 10, as they were in the initial cluster analysis. T h e cluster analysis using Ward's linkage m e t h o d uncovered t h r e e relatively large substance use groups and one small but distinct g r o u p o f substance users that were strikingly similar to the f o u r clusters f o u n d in Study 1. T h e g r o u p s were identified based o n the examination o f the d e n d r o g r a m as in Study 1. Descriptive labels a n d n u m b e r o f participants in each cluster were: low alcohol, N = 63; alcohol use, N = 123; alcohol and marijuana use, N = 99; and multiple substance use, N = 8. A g r a p h o f the pattern o f means for these f o u r substance use g r o u p s is depicted in Figure 2. T o externally validate the f o u r substance use g r o u p s identified in the cluster analysis, f o u r one-way MANOVAs were c o n d u c t e d . In each MANOVA, the indep e n d e n t variable was substance use g r o u p with the same f o u r levels used as those in Study 1: low alcohol use, alcohol use, alcohol and marijuana use, multiple substance use. T h e sets o f d e p e n d e n t variables for each MANOVA also were the same as those in Study 1 that e x a m i n e d (1) substance use network, (2) psychosocial functioning, (3) cocaine habit acquisition, and (4) self-efficacy.

Substance Use Clusters

Table 2.

57

Means of Dependent Variables Measured for Cluster Groups: Study 2

Low Use

Alcohol Use

Alcohol & Marijuana Use

Multiple Substance Use

p

P r e s s u r e to Use Alcohol P r e s s u r e to U s e M a r i j u a n a P r e s s u r e to Use D r u g s Alcohol U s e N e t w o r k D r u g Use N e t w o r k

2.66 1.32 I. 10 9.48 6.68

3.1 l 1.72 1.15 8.98 7.20

3.41 2.50 1.47 10.32 8.45

2.50 2.93 2.33 13.63 9.38

** **** *** ** ****

P s y c h o s o c i a l Factors Self-Derogation P u r p o s e in Life Suicide I d e a t i o n

1.70 3.81 1.27

1.65 3.95 1.23

1.82 3.74 1.39

2.48 3.01 2.25

** **** ****

1.79 1.32 1.18

2.75 1.80 1.42

2.98 2.10 1.57

2.85 2.49 2.50

**** **** ****

4.73 I. 11 1.03

4.73 1.20 1.10

4.07 1.74 1.29

2.41 2.36 2.23

*** **** ***

Variable Peer/Family Influence

Alcohol Temptations In Social Situations In Negative Situations O u t o f Habit

Cocaine Habit Acquisition Precontemplation Decision-making Maintenance *p < .05;

**p < .01;

***p < .001;

* * * * p < .0001.

M A N O V A 1: S u b s t a n c e U s e N e t w o r k

The first MANOVA examined group differences on reported substance use network. This involved five dependent measures: alcohol use network, drug use network, and perceived peer pressure to use alcohol, marijuana, and other drugs. Group means and significance levels for each of the dependent variables in Study 2 are presented in Table 2. T h e overall analysis on substance use network was significant, Wilks's k = .69, F(15,782) = 7.44, p < .0001, accounting for 31% of the variance in the dependent measures. Group differences were found among participants' reported family histories of alcohol problems, F(3, 290) = .4.73, p < .003. Multiple substance users reported a greater family history of alcohol problems than those who reported nearly no substance use and those who used primarily alcohol. Significant group differences were found for history of drug use, F(3, 290) = 6.99, p < .0001. Those using primarily alcohol and marijuana reported more family and friends with drug use problems than did those in the low use and those in the alcohol use groups. However, the size of the drug use network for the alcohol and marijuana use group did not differ from those in the multiple substance use group. Peer pressure to use alcohol also was significant across groups, F(3, 290) = 4.23, p < .002. Participants in the alcohol and marijuana use group reported

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more pressure to drink than did individuals in the low use group. Pressure to use marijuana was significant, F(3,290) = 26.25, p < .0001. In this case, the minimal use group experienced less pressure to use marijuana than did those in the other three groups, and the alcohol use group experienced less pressure to use marij u an a than did the alcohol and marijuana use and the multiple substance use groups. Pressure to use other illicit drugs was significant among the groups as well, F(3,290) = 14.21, p < .0001. Individuals in the alcohol and marijuana use and in the multiple substance use groups differed significantly from those in the other two groups, reporting that they experienced more pressure to use drugs other than alcohol or marijuana.

MANOVA 2: Psychosocial Variables Investigating group differences on the psychosocial variables of self-derogation, purpose in life, and suicide ideation yielded a significant overall MANOVA, F(9, 694) = 4.86, p < .0001, Wilks's ~ = .86, accounting for 14% of the variance. Univariate tests revealed that substance use groups differed on each of the three dependent variables: suicide ideation, F(3, 290) = 9.52, p < .0001; self-derogation, F(3, 290) = 5.04, p < .002; and purpose in life, F(3, 290) = 11.26, p < .0001. Participants in the multiple substance group reported more suicide ideation and self-derogation, and less purpose in life than those in the other three groups. In the case o f purpose in life, those in the alcohol and marijuana group also reported less purpose in life than did those in the alcohol use group.

MANOVA 3: Cocaine Habit Acquisition A third MANOVA examined scores on three reported stages of cocaine habit acquisition in accordance with Stern, Prochaska, Velicer, and Elder's (1987) model for habit acquisition. T h e analysis was significant, indicating that the groups could be differentiated on stages of cocaine habit acquisition, F(9, 694) = 12.97, p < .0001, Wilks's k = .68, accounting for 32% of the variance. T h e univariate test for group differences on Precontemplation was significant, F(3, 290) = 23.74,/7 < .0001. Individuals in the low use group reported no intention to ever use cocaine; they would be considered Precontemplators with regard to acquiring a cocaine habit and differed from all other groups on this measure. T h e alcohol use group also reported less of an intention to use cocaine than did those in the alcohol and marijuana use group and in the multiple substance use group. Finally, those in the alcohol and marijuana use group had less of an intention to use cocaine than did those in the multiple substance use group. T h e substance use groups also differed on Decision Making stage scores F(3, 290) = 27.13, p < .0001. Individuals in this stage have actually had some experience with cocaine and may be contemplating regular cocaine use but have not actually begun to use regularly. Tukey tests indicated that the multiple substance Use group and the alcohol and marijuana use group reported having tried cocaine more than did the other two groups, a n d the multiple substance use group had more experience than those in the alcohol and marijuana use group. Lasdy, in examining stages of cocaine habit acquisition, individuals differed at the g r o u p level on their Maintenance scores, F(3,290) = 17.17, p < .0001. Again

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the multiple substance use and the alcohol and marijuana use groups had h i g h e r scores on the Maintenance subscale, r e p o r t i n g no intention to stop using cocaine. T h e multiple substance use g r o u p scored h i g h e r on this measure, as they did on the Decision Making subscale, c o m p a r e d to those in the alcohol and marij u a n a use group, suggesting that they had less intention o f giving up their cocaine use. M A N O V A 4: A l c o h o l Temptations T h e f o u r t h MANOVA, which assessed temptations to use alcohol in a variety o f situations, was significant, F(9, 694) = 21.07, p < .0001, Wilks's k = .56, acc o u n t i n g for 44% o f the variance. G r o u p differences were detected at the univariate level for temptations to d r i n k in social situations, F(3, 290) = 16.45, p < .0001. T h e multiple substance use, alcohol and marijuana use, and alcohol use g r o u p s did not d i f f e r f r o m each o t h e r in their account o f being t e m p t e d to d r i n k in social situations; however, these three groups d i f f e r e d f r o m the low use g r o u p , who indicated that social situations were not a t e m p t a t i o n for drinking. T e m p t a t i o n to d r i n k in negative situations also was significant, F(3,290) = 16.45, p < .0001. Individuals in the multiple substance use g r o u p claimed to be m o r e t e m p t e d to d r i n k in negative situations than those in the alcohol and marijuana use group, who r e p o r t e d m o r e temptations than individuals in the alcohol use and minimal use groups. Additionally, the alcohol use g r o u p had h i g h e r scores on this m e a s u r e than did individuals in the minimal use group. Analyzing rep o r t e d temptations to d r i n k out o f habit also revealed significance, F ( 3 , 2 9 0 ) = 20.58, p < .0001. T h e results indicated that the alcohol use, alcohol and marij u a n a use, a n d multiple substance use groups were m o r e likely to d r i n k out o f habit than the low use group. T h e multiple substance use g r o u p also was m o r e likely to d r i n k out o f habit than the alcohol and marijuana use and the alcohol use groups. T h e results f r o m these f o u r sets o f analyses indicate that participants can be d i f f e r e n t i a t e d according to substance use typologies and patterns o f use. It was f o u n d that individuals were characteristically d i f f e r e n t when e x a m i n e d on a variety o f o u t c o m e measures based on their r e p o r t e d substance use. F u r t h e r m o r e , it was f o u n d that the m o r e involved individuals were with substances, the m o r e likely they were to have r e p o r t e d substance use a m o n g their networks o f family and friends, to f u n c t i o n less well psychologically, to be acquiring a cocaine habit, and to have less self-efficacy in a variety o f d r i n k i n g situations. T h e results were largely replicated across both studies, and where discrepancies occurred, trends most o f t e n were in the same direction. DISCUSSION

T h e findings f r o m this study suggest that individuals can be classified into o n e o f f o u r substance use g r o u p s based on the frequency, intensity, and a m o u n t o f substances used, and thus the hypothesis asserting that substance use typologies could be detected using the cluster analysis p r o c e d u r e was s u p p o r t e d . Across two i n d e p e n d e n t samples, the g r o u p s showed significant differences on a n u m -

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ber of variables, and inspection of the four clusters identified in each of the samples yielded high consistency in the overall cluster compositions. Each of the identified clusters was similar in size with respect to the number of participants in the two samples used in the analysis, with the exception of the low use and the alcohol and marijuana use groups: The former was larger in Study 1 and the latter was larger in Study 2. The reported patterns of use were highly similar, as were the results on the dependent variables on which the groups were differentiated. The behavior of the two linkage methods (Ward's and Average) was similar in both studies for both samples, although groups identified by the Ward's method were more well defined than groups identified by the Average linkage method. Average linkage identified four groups but tended to assign most users to either of the first two groups. This difference in outcome for the two methods is best explained as an artifact of the methods employed rather than as a result attributable to the data. Based on these grouping criteria, Ward's method is more likely to identify a larger number of small clusters within a data set than the Average linkage method, which tends to identify a fewer number of large clusters (Lorr, 1987). The results will now be discussed on the basis of the four groups identified using Ward's linkage method. The findings in this study supported the hypothesis that tested social learning theory utilizing measures of peer and family influences. Perceived peer pressure was significantly associated with reported substance use. This was particularly apparent in both data sets for pressure to use alcohol, marijuana, and other illicit substances. It was predominantly the multiple substance users who consistently reported experiencing greater amounts of peer pressure, followed by the marijuana and alcohol users. Individuals in the low alcohol use group who reported using no other substances reported the least amount of peer pressure. This is not surprising because the substances most likely were not used in their circle of acquaintances. Thus, once one's peer group involves the use of several substances of any kind, the opportunity and pressure to use the substances would be more probable, and this, hence, supports social learning theory. Within the alcohol use group, individuals did not report as much pressure to use alcohol as did other groups, even though they used alcohol heavily. This finding makes sense given that the sample is comprised of college students, and drinking is considered the norm among the majority of students. When a behavior is normative, it seems less likely that individuals would feel pressured by their peer group to engage in that behavior because they most likely intend to do so. This inference can similarly be applied to self-efficacy and temptations to drink in social situations (addressed later in this discussion). One finding of interest in assessing social learning theory and substance use was the lack of overall support for alcohol and other drug family histories across both samples. A weak potential relationship was found in Study 1. It was expected that these variables would have been significant among the heavier substance users, and they were in Study 2. It seems reasonable to have expected students in the multiple substance use groups to identify alcohol and/or other drug problems in their families because it is often the case that individuals take

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on the characteristics of their parents, as social learning theory would suggest. It may even be that some individuals have parents with substance use problems; however, the answers to these questions were based on the students' self-reports regarding their perception of whether or not their parents had a substance problem. Their report of their parent's substance use clearly could be assessed relative to their own heavy substance use; admitting that their parents had a problem would have required that they admit their own problem. This point underscores the need for education regarding substance use problems in our society. It is all too common to not perceive someone as having trouble with alcohol or other substances simply because they do not fit a stereotypical image of what an "alcoholic" or drug misuser looks like (Willoughby, 1979). Given the tendency for individuals to underreport sensitive phenomena on survey instruments, and the need in our society for education regarding the many areas of life that can be affected by substances, it is not surprising that consistent results were difficult to detect on this measure even among those in the subgroup for whom family history problems with substances was most likely. Additional findings suggested that multiple substance users suffer from more psychological distress. As with the family history variables, the associations for these variables were weak in Study 1 and more pronounced in Study 2 - something that was anticipated for these variables. These results coincide with those reported by Kandel (1982), who found that the psychosocial factor selfesteem was less stable and reliable than peer influence. This is also in accordance with the findings of Brennan, Walfish, and AuBuchon (1986a), who determined that personality variables were weak but consistent and should be examined with other variables rather than in isolation. The one psychosocial variable that was clearly identified in both studies was the incidence of suicide ideation among the heavier substance users. These individuals apparently consider suicide, have morbid thoughts, and hold the notion that their lives may end in suicide more often than do individuals in the other three clusters. In Study 2, the multiple substance use group reported poorer psychosocial functioning on all three psychosocial measures (suicide ideation, self-derogation, and purpose in life). The connection between the significance in Study 2 on these and the family history variables and the multiple substance use group may lie in the fact that the group in Study 2 also had higher substance use means than did those in Study I. Hence, these results are congruent with those found thus far indicating a pattern of decreased functioning with increased substance use. This might lead one to conclude that the multiple substance users, particularly those in Study 2, are more like individuals found in a clinical population (Minugh et al., 1992). T h e results for perceived lack of self-efficacy as measured by temptations to drink alcohol were similar in both studies. Across the studies, the alcohol use group reported fewer temptations to drink socially than did the polysubstance use groups. This result is interesting because one might suspect that the alcohol use group would be more tempted to drink in social situations simply because of their more frequent use. Alcohol consumption is fairly normal behavior on college campuses. Hence, a research project and sample of this size would probabilistically include a large group of heavy alcohol drinkers. For these individuals,

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drinking at a social gathering or when relaxing with friends is most likely perceived as a normal behavior rather than as a temptation, in contrast to individuals who rarely drink or who may have more experience with other substances and hence more frequent exposure to other stimulus cues for enticing drinking. Temptations to drink in unpleasant situations (e.g., feeling depressed, angry,. etc.) were nearly similar for both samples, showing higher levels of temptation in these circumstances for heavier and more frequent substance users. This finding is congruent with that of Kandel (1982), who found that depressed mood was reliably related to substance use, and with that of Harlow and Minugh (1989), who found that cocaine users were more likely to experience temptations to drink in unpleasant situations and out of habit. The more substances used, and the more frequently they were used, the more participants tended to report temptations to drink in unpleasant situations. Drinking out of habit also was reported as more tempting for individuals in the substance use groups using alcohol and other drugs. Historically, philosophies of habit primarily center on the notion of repetition and adaptation (e.g., James, Lamark, & Darwin in Leahey, 1992). If the findings for both temptations to drink out of habit and temptations to drink in unpleasant situations are viewed as a process in which relief is provided for psychological distress, then drinking for some may be viewed as a means for adapting to life's stressors. This inference is in accordance with Willoughby's (1979) learned behavior model of alcohol and other substance troubles, and other models supporting coping skills as mediators in the process of relapse prevention (Marlatt, 1985). These approaches suggest that drinking alcohol is often a coping mechanism. Individuals' scores on the habit and negative affect situation subscales would suggest that those who have high scores on both measures may be drinking habitually in an attempt at adaptation, albeit an inadequate one. This pattern was particularly evident for multiple substance users, who had the highest reported scores on the habit and negative situation temptation measures. Participants who reported polydrug use also scored high on the measure assessing the likelihood that one is in the process of acquiring a cocaine habit. This finding, in both samples, is in line with earlier findings by Harlow and Minugh (1989); individuals in the cluster whose substance use was to a great degree limited to alcohol use were found to be in the Precontemplation stage, indicating no intention of beginning cocaine use. In both studies, the low alcohol use group indicated they had no intentions of using cocaine and were differentiated from the multiple substance and the alcohol and marijuana users, who had more experience using cocaine. Although the alcohol and marijuana users reported more experimentation than the two alcohol groups, they reported less than the multiple substance use group. This also was the case in Study 2, with the greatest distinction in terms of the Precontemplation stage existing between the alcohol use group who reported no intentions of beginning cocaine use and the alcohol and marijuana use group who reported a small amount of experimentation, indicating a willingness to try cocaine and possibly begin regular use. Multiple substance users' scores on this scale placed them in the Maintenance stage of habit acquisition. Their scores indicated that, at this time, they had no intention

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of quitting their cocaine use. This information is useful in defining the fourth cluster as a group that experienced problems in living, although they had not yet reached the point in their use that would lead them to think they needed to quit. These results are not surprising (i.e., someone using cocaine would logically have scores in a direction indicative of habit development), but they provide more information on the multiple substance use and the alcohol and marijuana use groups in terms of their intentions with respect to the cocaine use. In particular, the findings provide support for Kandel's stage theory of substance use (Kandel, 1975; Kandel et al., 1992). In this case, marijuana seems to be the transition drug for cocaine experimentation and use. As noted by Kandel (personal communication, 1992) and Willoughby (1979), being in one stage of substance use does not necessarily imply one will progress to the next stage. An individual may reach a particular stage and stay in that stage for life, regress, or quit entirely, thus not advancing to another stage. However, individuals who report using illicit drugs typically report experience in earlier stages of drug involvement, as was the case in this study (Kandel, 1975; Kandel et al., 1992). T h e overall results of this study clearly indicate that substance users differ from each other in terms of the types of substances used and the frequency and intensity of their substance use on a variety of measures. These findings should be analyzed in terms of their context--that is, college students are quite possibly uncharacteristic of society at large, and of long-term substance users, because they have had less time to experience trouble directly associated with their substance use. This study is not without its limitations. Most important, its greatest shortcoming is undoubtedly the small cell sizes found for the multiple substance use groups in both studies. However, these results are consistent with those found in college samples in previous studies using similar variables in a cluster analysis (Clifford, Edmundson, Koch, & Dodd, 1989), wherein smaller numbers were found for groups engaging in frequent illicit substance use and larger numbers clustering in groups reporting near abstinence and heavy, frequent drinking. It can reasonably be concluded that the clusters are largely representative of what one would expect to find in examining a college sample. Future Directions

It is recommended that research of this type be taken into inner cities and treatment centers to determine whether the findings and speculations from this study hold in more heterogeneous samples. It also would be interesting to determine whether inferences made regarding an escalation of problems in living would be more clear in clusters with more frequent and intense substance use, even though causal inferences could not be made. The implications of the study might be extended to settings in which substance abuse education is taught (e.g., schools, workplaces). These findings lend support to the notion that peer pressure is a significant factor related to substance abuse, that substance use appears to escalate in a series of stages for most substance users who have used a variety of substances, and that self-efficacy is an important factor related to perceived

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b e h a v i o r a l e x p e c t a t i o n s with r e g a r d to s u b s t a n c e use. It is i m p o r t a n t to stress to individuals b e i n g e d u c a t e d r e g a r d i n g s u b s t a n c e use p r o b l e m s that all individuals are n o t alike. A l t h o u g h causality c a n n o t be i n f e r r e d , it is i m p o r t a n t to n o t e that p r o b l e m s in living a n d s u b s t a n c e use s e e m to be related, b u t it r e m a i n s u n c l e a r w h i c h is the a n t e c e d e n t a n d which is the c o n s e q u e n t . T h u s , s u b s t a n c e a b u s e e d u c a t i o n m i g h t a d d r e s s this relationship, n o t i n g that individuals e i t h e r u s i n g substances a n d / o r e x p e r i e n c i n g psychosocial distress m i g h t be at risk f o r substance use p r o b l e m s . It is e x p e c t e d that r e s e a r c h a m o n g individuals in t r e a t m e n t will f u r t h e r valid a t e the f i n d i n g s o f this study. I f this is the case, f i n d i n g s o f this n a t u r e w o u l d f u r t h e r s u p p o r t the original assertion o f this study that individuals w h o use substances s h o u l d be e x a m i n e d s e p a r a t e l y a c c o r d i n g to s u b s t a n c e use type a n d patt e r n s a n d n o t viewed as o n e u n i q u e set o f individuals in society s h a r i n g identical characteristics. REFERENCES

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