Internal consistency of the Seasonal Pattern Assessment Questionnaire (SPAQ).

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 = 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.