Background Few data exist on the psychosocial factors associated with attrition in longitudinal surveys. This study was undertaken 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. Data were analysed using multilevel logistic regression.
Results 7777 primary care attendees aged 18–75 years were selected, of whom 1251 (16.1%) were excluded. Of the remaining 6526, 1084 (16.6%) refused to participate. Thus, 5442 patients (attending 231 family physicians in 41 health centres) were interviewed at baseline, of whom 3804 (70%) and 3567 (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. There were different profiles for the patients lost at 12 months when predictors measured at baseline versus 6 months were included.
Conclusions These findings suggest that 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, indicating that different retention strategies should be applied for patients interviewed at 6 and 12 months.
- Longitudinal studies
- patient dropouts
- primary healthcare
- psychosocial factors
- selection bias
- bias ME
- multilevel models
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Funding This work was supported in Spain by grants from the Spanish Ministry of Health (grant FIS references: PI041980, PI041771, PI042450 and PI06/1442); the Andalusian Council of Health (grant references: 05/403 and 06/278); the Spanish Network of Primary Care Research ‘redIAPP’ (RD06/0018), the ‘Aragón group’ (RD06/0018/0020), the ‘Baleares group’ (RD07/0018/0033) and the ‘SAMSERAP group’ (RD06/0018/0039). The Malaga sample, as part of the predictD-International study, was also co-funded by a grant from The European Commission (reference QL4-CT2002-00683).
Competing interests None.
Ethics approval Ethical approval for the study was obtained from the relevant ethics committees.
Provenance and peer review Not commissioned; externally peer reviewed.