P1-409 Differential loss of participants does not necessarily cause selection bias

Department of Public Health, University of Otago, New Zealand.
Australian and New Zealand Journal of Public Health (Impact Factor: 1.98). 06/2012; 36(3):218-22. DOI: 10.1111/j.1753-6405.2012.00867.x
Source: PubMed


Most research is affected by differential participation, where individuals who do not participate have different characteristics to those who do. This is often assumed to induce selection bias. However, selection bias only occurs if the exposure-outcome association differs for participants compared to non-participants. We empirically demonstrate that selection bias does not necessarily occur when participation varies in a study.
We used data from three waves of the longitudinal Survey of Family, Income and Employment (SoFIE). We examined baseline associations of labour market activity and education with self-rated health using logistic regression in five participation samples: A) the original sample at year one (n=22,260); B) those remaining in the sample (n=18,360); C) those (at year 3) consenting to data linkage (n=14,350); D) drop outs over three years (n=3,895); and E) those who dropped out or did not consent (n=7,905).
Loss to follow-up was more likely among lower socioeconomic groups and those with poorer health. However, for labour market activity and education, the odds of reporting fair/poor health were similar across all samples. Comparisons of the mutually exclusive samples (C and E) showed no difference in the odds ratios after adjustment for sociodemographic (participation) variables. Thus, there was little evidence of selection bias.
Differential loss to follow-up (drop out) need not lead to selection bias in the association between exposure (labour market activity and education) and outcome (self-rated health).

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    • "However, unless these dropout rates were jointly distributed by the social and economic exposure measures and drinking behavior outcome measures, the effect of attrition on the studied associations is likely to be minimal. We have also previously shown that for the association of employment and education with self-rated health there appears to be no difference in the association among those respondents leaving the SoFIE study (non-participants) compared to those respondents who stayed in the study (participants) [36]. "
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    • "Limitations include absence of information on psychological factors, such as diagnosed mental health conditions or psychological distress: this limits our ability to consider such conditions as covariates or effect modifiers, but our model is robust whether or not the mental health component of QoL is included, suggesting this is not a major issue. Additionally, the initial response rate of 57%, while lower than desirable, is similar to other comparable Australian studies, [19] and a lower response rate does not mean that relationships between outcome and exposure are necessarily biased [28]. Participants who did not continue in the study were older, heavier, with pain at more sites at baseline than the remaining participants. "
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