Objective: Some researchers have reported that distribution of total depressive symptom scores in the general population may follow an exponential pattern except at the lowest end of the scores. To understand the mechanism responsible for this phenomenon, we investigated the mathematical patterns of the individual distributions for each item of a depressive symptom scale.
Methods: We analysed data from 32,022 participants in the general population who participated in the Active Survey of Health and Welfare, Japan. Depressive symptoms were assessed using the Japanese version of Center for Epidemiologic Studies Depression Scale (CES-D). CES-D has 20 items, each of which is scored in 4 grades: 'Rarely', 'Some', 'Much' and 'Most of the time'.
Results: The individual distributions of 16 negative items belonging to the depressive mood, somatic symptoms and retarded activities, and interpersonal relations categories, followed a common mathematical pattern, which displayed different distributions with a boundary at 'Some'. The distributions for the 16 items between 'Rarely' and 'Some' appeared to cross at a single point. On the other hand, the distributions of the 16 items between 'Some' and 'Most' followed a linear pattern when plotted using a log-normal scale. The remaining 4 items in the positive affect subscale showed non-specific patterns.
Conclusions: The common mathematical pattern of the 16 negative item distributions may contribute to the exponential pattern of the distribution of total depressive symptom scores except at the lowest end of the scores.
Keywords: EPIDEMIOLOGY; STATISTICS & RESEARCH METHODS.
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