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Internal consistency of the Seasonal Pattern Assessment Questionnaire (SPAQ)
Journal of Affective Disorders 42 (1997) 113–116
Research report
Internal consistency of the Seasonal Pattern Assessment Questionnaire (SPAQ) Andres Magnusson*, Svein Friis, Stein Opjordsmoen ˚ University Hospital, 0407 Oslo, Norway Department of Psychiatry, Ulleval Received 8 July 1996; revised 4 October 1996; accepted 4 October 1996
Abstract We examined the consistency reliability of the widely used Seasonal Pattern Assessment Questionnaire (SPAQ). In total, 587 questionnaires from a random sample of the general population were analysed. The 6 items of the Seasonality Scale Index (SSI) correlated fairly well, and consequently this scale proved to have a high internal consistency (alpha 5 0.82). Items from the SSI that measure seasonal variations in sleep and weight were consistent with other SPAQ scales that also measure these dimensions. Since the questionnaires analysed were from an epidemiological study, the high consistency reliability shows that the symptoms probed for by the SSI, tend to cluster in susceptible individuals in the general population. 1997 Elsevier Science B.V. Keywords: Seasonal affective disorder; Seasonal Pattern Assessment Questionnaire; Reliability; Factor analysis
1. Introduction The Seasonal Pattern Assessment Questionnaire (SPAQ), developed by (Rosenthal et al., 1987) is a widely used instrument to investigate Seasonal Affective Disorder (SAD). Several reports, based on this questionnaire, have been published during the last years (Magnusson, 1996). The central feature of the SPAQ is the six-item Seasonality Score Index (SSI). The test-retest reliability of the SSI has been shown to be good (Thompson et al., 1988; Hardin et al., 1991; Marriott, 1993; Murray et al., 1993; WirzJustice et al., 1993), but we are not aware of any studies on the internal consistency reliability of the questionnaire. *Corresponding author.
The aim of the present study was to examine: 1. What are the intercorrelations between the SSIitems? 2. What is the factor structure of the SSI-items? 3. What is the internal consistency of the SSI-scale? 4. Are the seasonal variations in sleep and weight items from the SSI consistent with other SPAQitems that also measure these dimensions?
2. Material and methods
2.1. The questionnaire The Seasonal Pattern Assessment Questionnaire (SPAQ) is an instrument to investigate mood and behavioural changes with the seasons. It has several
0165-0327 / 97 / $17.00 1997 Elsevier Science B.V. All rights reserved PII S0165-0327( 96 )00104-8
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A. Magnusson et al. / Journal of Affective Disorders 42 (1997) 113 – 116
scales that measure for example variation in body weight with seasons and how long the respondents sleep in summer, winter, autumn and spring. The central feature of the SPAQ is the six-item SSI that measure seasonal variations in mood, appetite, weight, sleep, energy and socialising. Each item is scored on a five point scale, ranging from 0 (no change) to 4 (extremely marked change). The questionnaire was provided by Dr. Norman E. Rosenthal at the National Institute of Mental Health. It was translated into Icelandic and then back-translated to English to ascertain that the translation was accurate.
2.2. Sampling The questionnaires analysed in this study were part of a community epidemiological study of SAD in Iceland, which is a typical Nordic country. The details of the sampling procedures have previously been described (Magnusson and Stefansson, 1993). Briefly, the SPAQ was mailed to 1000 individuals, 17–67 years old, randomly selected from the Icelandic National Register in spring 1988. The response rate was 61%.
3. Results The inter-correlation coefficients of the six SSI items ranged from 0.24 to 0.65 (Table 1). The weight and appetite items were strongly intercorrelated, but they had a weaker correlation with the other items. This was also revealed in a factor analysis. As shown in Table 2, the items of weight and appetite seemed to constitute a separate factor, while the other four items loaded strongly on the first factor. Factor 1 has a high eigenvalue (3.14), and explains 52.4% of the variance. The second factor has a much lower eigenvalue (1.06), and although it explains additional 17.7% of the variance, it is uncertain whether it is justified to group these two items in a separate subscale. As shown in Table 3, the internal consistency was high. The corrected item-scale correlation ranged from 0.47 to 0.72. These correlations were not clearly lower for weight and appetite than for the other items. This indicates
Table 2 A principal component varimax factor analysis of the 6 items from the Seasonality Score Index. Break of point is eigenvalue . 1; n 5 572 Seasonal variation in:
2.3. Data processing The statistical calculations were made by SPSS for Windows, 6th version. In total 587 questionnaires were analysed. Due to missing values, 15 to 29 questionnaires, depending on the statistical test used, were excluded from the analyses.
Sleep Social activity Mood Weight Appetite Energy
Varimax rotated factor loading Factor 1
Factor 2
0.72 0.75 0.85 0.13 0.29 0.77
0.17 0.14 0.13 0.91 0.84 0.36
Table 1 The correlation coefficients between the individual items of the Seasonality Score Index (n 5 558) Seasonal variation in:
Sleep Social activity Mood Weight Appetite Energy
Correlation matrix Sleep
Social activity
Mood
Weight
Appetite
Energy
1.00 0.36 0.48 0.24 0.35 0.54
1.00 0.56 0.24 0.35 0.48
1.00 0.27 0.34 0.65
1.00 0.63 0.40
1.00 0.47
1.00
A. Magnusson et al. / Journal of Affective Disorders 42 (1997) 113 – 116
115
Table 3 Internal consistency analysis of the Seasonality Scone Index (n 5 558) Seasonal variation in:
Item mean
Standard deviation
Alpha if item deleted
Corrected item-scale correlation
Sleep Social activity Mood Weight Appetite Energy
1.20 0.59 1.16 0.69 0.61 1.27
0.95 0.93 1.07 0.86 0.88 1.11
0.80 0.80 0.77 0.81 0.79 0.76
0.54 0.55 0.65 0.47 0.57 0.72
Internal consistency is 0.82 (Cronbach’s alpha).
that the 6 items constitute a coherent, one dimensional scale, which is further supported by a high Cronbach’s alpha.
3.1. Seasonal changes in sleep and weight In two of the items of the SSI, the subjects are asked to grade their seasonal variations in sleep and body weight from ‘no variations’ to ‘extremely marked variations’. Other scales on the SPAQ ask how many hours the respondents sleep in spring, summer, autumn and winter and another scale inquires how many kilos the respondents’ weight fluctuates during the course of a year. The correlation between the seasonal variation in sleep-item of the SSI, and the difference in hours slept in winter vs. summer, was 0.62 (n 5 572, P , 0.001). The correlation between the seasonal variation in weightitem of the SSI and the weight fluctuation in kilos during the course of a year scale, was 0.59 (n 5 564, P , 0.001).
4. Discussion To our knowledge, the present study is the first to examine the SPAQ’s internal consistency reliability. We found a good internal consistency for the 6 items of the SSI as reflected in a high Cronbach’s alpha. This indicates that the index is well constructed and that the 6 different items are coherent and act synergistically to measure the underlying concept. Furthermore, because the analyses are based on a large number of questionnaires randomly sampled from a general population, the high internal consis-
tency implies that seasonal changes in sleep, social activity, mood, weight, appetite and energy, have a tendency to co-exist in the general population. Factor analysis of the SSI suggested a two factor solution. However, the eigenvalue of the second factor was relatively low, the SSI items were coherent, and the internal consistency reliability of the SSI would not be improved if the weight or appetite items were deleted from it (Table 3, column 3). We thus conclude that a two-factor solution is doubtful.
4.1. Seasonal changes in weight and sleep There was a high correlation for seasonal variation in weight measured by the two different scales on the SPAQ. Similarly seasonal variation in sleep, measured by two different scales, also showed a high correlation. This indicates that these two dimensions can be reliably measured by the SPAQ. All six items in the SSI have an identical format. This could predispose to scale effects and systematic error which might lead to an increase in the estimated reliability. However, we found that the seasonal variation in sleep and weight items from the SSI correlated strongly with other SPAQ scales of these measures with different formats and in different contexts. This argues against any substantial scale effect. In summary, this study found that the SSI is well constructed and reliable. Items from the SSI that measure seasonal variations in sleep length and weight were consistent with other SPAQ items that also measure these dimensions. The high consistency of the SSI items in an epidemiological survey indicates that a cluster of symptoms tend to co-occur
116
A. Magnusson et al. / Journal of Affective Disorders 42 (1997) 113 – 116
in the general population. These constitute a syndrome of seasonal variation in mood and behaviour, and SAD is probably the most extreme form of this.
Acknowledgments We would like to thank Oddmar Moen for help with the statistics.
References Hardin, T.A., Wehr, T.A., Brewerton, T., Kasper, S., Berrettini, W., Rabkin, J. and Rosenthal, N.E. (1991) Evaluation of seasonality in six clinical populations and two normal populations. J. Psychiatr. Res. 25(3), 75–87. Magnusson, A. (1996) Validation of the seasonal pattern assessment questionnaire (SPAQ). J. Affect. Disord. (in press).
Magnusson, A. and Stefansson, J.G. (1993) Prevalence of seasonal affective disorder in Iceland. Arch. Gen. Psychiatry 50(12), 941–946. Marriott, P.F. (1993) An assessment of SPAQ and SPAQ 1 reliability. Bulletin of the society for light treatment and biological rhythms 5(3), 33. Murray, G., Armstrong, S. and Hay, D. (1993) SPAQ reliability in an Australian twin sample. Bull. Soc. Light Treatment Biol. Rhythms 5(3), 32. Rosenthal, N.E., Genhart, M.J., Sack, D.A., Skwerer, R.G. and Wehr, T.A. (1987) Seasonal affective disorder and its relevance for the understanding and treatment of bulimia. In Hudson, J.I. and Pope, H.G. (Eds.), The Psychobiology of Bulimia. American Psychiatric Press, Washington, DC, pp. 205–228. Thompson, C., Stinson, D., Fernandez, M., Fine, J. and Isaacs., G. (1988) A comparison of normal, bipolar and seasonal affective disorder subjects using the Seasonal Pattern Assessment Questionnaire. J. Affect. Disord. 14(3), 257–264. Wirz-Justice, A., Graw, P. and Recker, S. (1993) The Seasonal Pattern Assessment Questionnaire (SPAQ): Some comments. Bull. Soc. Light Treatment Biol. Rhythms 5(3), 31.
Research report
Internal consistency of the Seasonal Pattern Assessment Questionnaire (SPAQ) Andres Magnusson*, Svein Friis, Stein Opjordsmoen ˚ University Hospital, 0407 Oslo, Norway Department of Psychiatry, Ulleval Received 8 July 1996; revised 4 October 1996; accepted 4 October 1996
Abstract We examined the consistency reliability of the widely used Seasonal Pattern Assessment Questionnaire (SPAQ). In total, 587 questionnaires from a random sample of the general population were analysed. The 6 items of the Seasonality Scale Index (SSI) correlated fairly well, and consequently this scale proved to have a high internal consistency (alpha 5 0.82). Items from the SSI that measure seasonal variations in sleep and weight were consistent with other SPAQ scales that also measure these dimensions. Since the questionnaires analysed were from an epidemiological study, the high consistency reliability shows that the symptoms probed for by the SSI, tend to cluster in susceptible individuals in the general population. 1997 Elsevier Science B.V. Keywords: Seasonal affective disorder; Seasonal Pattern Assessment Questionnaire; Reliability; Factor analysis
1. Introduction The Seasonal Pattern Assessment Questionnaire (SPAQ), developed by (Rosenthal et al., 1987) is a widely used instrument to investigate Seasonal Affective Disorder (SAD). Several reports, based on this questionnaire, have been published during the last years (Magnusson, 1996). The central feature of the SPAQ is the six-item Seasonality Score Index (SSI). The test-retest reliability of the SSI has been shown to be good (Thompson et al., 1988; Hardin et al., 1991; Marriott, 1993; Murray et al., 1993; WirzJustice et al., 1993), but we are not aware of any studies on the internal consistency reliability of the questionnaire. *Corresponding author.
The aim of the present study was to examine: 1. What are the intercorrelations between the SSIitems? 2. What is the factor structure of the SSI-items? 3. What is the internal consistency of the SSI-scale? 4. Are the seasonal variations in sleep and weight items from the SSI consistent with other SPAQitems that also measure these dimensions?
2. Material and methods
2.1. The questionnaire The Seasonal Pattern Assessment Questionnaire (SPAQ) is an instrument to investigate mood and behavioural changes with the seasons. It has several
0165-0327 / 97 / $17.00 1997 Elsevier Science B.V. All rights reserved PII S0165-0327( 96 )00104-8
114
A. Magnusson et al. / Journal of Affective Disorders 42 (1997) 113 – 116
scales that measure for example variation in body weight with seasons and how long the respondents sleep in summer, winter, autumn and spring. The central feature of the SPAQ is the six-item SSI that measure seasonal variations in mood, appetite, weight, sleep, energy and socialising. Each item is scored on a five point scale, ranging from 0 (no change) to 4 (extremely marked change). The questionnaire was provided by Dr. Norman E. Rosenthal at the National Institute of Mental Health. It was translated into Icelandic and then back-translated to English to ascertain that the translation was accurate.
2.2. Sampling The questionnaires analysed in this study were part of a community epidemiological study of SAD in Iceland, which is a typical Nordic country. The details of the sampling procedures have previously been described (Magnusson and Stefansson, 1993). Briefly, the SPAQ was mailed to 1000 individuals, 17–67 years old, randomly selected from the Icelandic National Register in spring 1988. The response rate was 61%.
3. Results The inter-correlation coefficients of the six SSI items ranged from 0.24 to 0.65 (Table 1). The weight and appetite items were strongly intercorrelated, but they had a weaker correlation with the other items. This was also revealed in a factor analysis. As shown in Table 2, the items of weight and appetite seemed to constitute a separate factor, while the other four items loaded strongly on the first factor. Factor 1 has a high eigenvalue (3.14), and explains 52.4% of the variance. The second factor has a much lower eigenvalue (1.06), and although it explains additional 17.7% of the variance, it is uncertain whether it is justified to group these two items in a separate subscale. As shown in Table 3, the internal consistency was high. The corrected item-scale correlation ranged from 0.47 to 0.72. These correlations were not clearly lower for weight and appetite than for the other items. This indicates
Table 2 A principal component varimax factor analysis of the 6 items from the Seasonality Score Index. Break of point is eigenvalue . 1; n 5 572 Seasonal variation in:
2.3. Data processing The statistical calculations were made by SPSS for Windows, 6th version. In total 587 questionnaires were analysed. Due to missing values, 15 to 29 questionnaires, depending on the statistical test used, were excluded from the analyses.
Sleep Social activity Mood Weight Appetite Energy
Varimax rotated factor loading Factor 1
Factor 2
0.72 0.75 0.85 0.13 0.29 0.77
0.17 0.14 0.13 0.91 0.84 0.36
Table 1 The correlation coefficients between the individual items of the Seasonality Score Index (n 5 558) Seasonal variation in:
Sleep Social activity Mood Weight Appetite Energy
Correlation matrix Sleep
Social activity
Mood
Weight
Appetite
Energy
1.00 0.36 0.48 0.24 0.35 0.54
1.00 0.56 0.24 0.35 0.48
1.00 0.27 0.34 0.65
1.00 0.63 0.40
1.00 0.47
1.00
A. Magnusson et al. / Journal of Affective Disorders 42 (1997) 113 – 116
115
Table 3 Internal consistency analysis of the Seasonality Scone Index (n 5 558) Seasonal variation in:
Item mean
Standard deviation
Alpha if item deleted
Corrected item-scale correlation
Sleep Social activity Mood Weight Appetite Energy
1.20 0.59 1.16 0.69 0.61 1.27
0.95 0.93 1.07 0.86 0.88 1.11
0.80 0.80 0.77 0.81 0.79 0.76
0.54 0.55 0.65 0.47 0.57 0.72
Internal consistency is 0.82 (Cronbach’s alpha).
that the 6 items constitute a coherent, one dimensional scale, which is further supported by a high Cronbach’s alpha.
3.1. Seasonal changes in sleep and weight In two of the items of the SSI, the subjects are asked to grade their seasonal variations in sleep and body weight from ‘no variations’ to ‘extremely marked variations’. Other scales on the SPAQ ask how many hours the respondents sleep in spring, summer, autumn and winter and another scale inquires how many kilos the respondents’ weight fluctuates during the course of a year. The correlation between the seasonal variation in sleep-item of the SSI, and the difference in hours slept in winter vs. summer, was 0.62 (n 5 572, P , 0.001). The correlation between the seasonal variation in weightitem of the SSI and the weight fluctuation in kilos during the course of a year scale, was 0.59 (n 5 564, P , 0.001).
4. Discussion To our knowledge, the present study is the first to examine the SPAQ’s internal consistency reliability. We found a good internal consistency for the 6 items of the SSI as reflected in a high Cronbach’s alpha. This indicates that the index is well constructed and that the 6 different items are coherent and act synergistically to measure the underlying concept. Furthermore, because the analyses are based on a large number of questionnaires randomly sampled from a general population, the high internal consis-
tency implies that seasonal changes in sleep, social activity, mood, weight, appetite and energy, have a tendency to co-exist in the general population. Factor analysis of the SSI suggested a two factor solution. However, the eigenvalue of the second factor was relatively low, the SSI items were coherent, and the internal consistency reliability of the SSI would not be improved if the weight or appetite items were deleted from it (Table 3, column 3). We thus conclude that a two-factor solution is doubtful.
4.1. Seasonal changes in weight and sleep There was a high correlation for seasonal variation in weight measured by the two different scales on the SPAQ. Similarly seasonal variation in sleep, measured by two different scales, also showed a high correlation. This indicates that these two dimensions can be reliably measured by the SPAQ. All six items in the SSI have an identical format. This could predispose to scale effects and systematic error which might lead to an increase in the estimated reliability. However, we found that the seasonal variation in sleep and weight items from the SSI correlated strongly with other SPAQ scales of these measures with different formats and in different contexts. This argues against any substantial scale effect. In summary, this study found that the SSI is well constructed and reliable. Items from the SSI that measure seasonal variations in sleep length and weight were consistent with other SPAQ items that also measure these dimensions. The high consistency of the SSI items in an epidemiological survey indicates that a cluster of symptoms tend to co-occur
116
A. Magnusson et al. / Journal of Affective Disorders 42 (1997) 113 – 116
in the general population. These constitute a syndrome of seasonal variation in mood and behaviour, and SAD is probably the most extreme form of this.
Acknowledgments We would like to thank Oddmar Moen for help with the statistics.
References Hardin, T.A., Wehr, T.A., Brewerton, T., Kasper, S., Berrettini, W., Rabkin, J. and Rosenthal, N.E. (1991) Evaluation of seasonality in six clinical populations and two normal populations. J. Psychiatr. Res. 25(3), 75–87. Magnusson, A. (1996) Validation of the seasonal pattern assessment questionnaire (SPAQ). J. Affect. Disord. (in press).
Magnusson, A. and Stefansson, J.G. (1993) Prevalence of seasonal affective disorder in Iceland. Arch. Gen. Psychiatry 50(12), 941–946. Marriott, P.F. (1993) An assessment of SPAQ and SPAQ 1 reliability. Bulletin of the society for light treatment and biological rhythms 5(3), 33. Murray, G., Armstrong, S. and Hay, D. (1993) SPAQ reliability in an Australian twin sample. Bull. Soc. Light Treatment Biol. Rhythms 5(3), 32. Rosenthal, N.E., Genhart, M.J., Sack, D.A., Skwerer, R.G. and Wehr, T.A. (1987) Seasonal affective disorder and its relevance for the understanding and treatment of bulimia. In Hudson, J.I. and Pope, H.G. (Eds.), The Psychobiology of Bulimia. American Psychiatric Press, Washington, DC, pp. 205–228. Thompson, C., Stinson, D., Fernandez, M., Fine, J. and Isaacs., G. (1988) A comparison of normal, bipolar and seasonal affective disorder subjects using the Seasonal Pattern Assessment Questionnaire. J. Affect. Disord. 14(3), 257–264. Wirz-Justice, A., Graw, P. and Recker, S. (1993) The Seasonal Pattern Assessment Questionnaire (SPAQ): Some comments. Bull. Soc. Light Treatment Biol. Rhythms 5(3), 31.