Background: Few data exist on the psychosocial factors associated with attrition in longitudinal surveys. We aim to determine psychosocial and sociodemographic predictors of attrition from a longitudinal study of the onset and persistence of episodes of major depression in primary care.
Methods: A systematic random sample of general practice attendees was recruited in seven Spanish provinces between October 2005 and February 2006. Major depression was diagnosed using the Composite International Diagnostic Interview and a set of 39 individual and environmental risk factors for depression were assessed at baseline and after 6 and 12 months of follow-up. We used multilevel logistic regression.
Results: We selected 7,777 primary care attendees aged 18-75 years, of whom 1,251 (16.1%) were excluded. Of the remaining 6,526, 1,084 (16.6%) refused to participate. Thus, 5,442 patients (attending 231 family physicians in 41 health centers) were interviewed at baseline, of whom 3,804 (70%) and 3,567 (66%) remained at 6 and 12 months of follow-up, respectively. The province and sociodemographic factors were stronger predictors of attrition than psychosocial factors. Depression and anxiety had no effect but other psychosocial factors affected attrition. We also found different profiles for the patients lost at 12 months when we included predictors measured at baseline versus 6 months.
Conclusions: These findings suggest several psychosocial factors might be considered factors of attrition in primary care cohorts and confirm that baseline characteristics are insufficient for analysing non-response in longitudinal studies, suggesting that different retention strategies should be applied for patients interviewed at 6 and 12 months.
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