Effects of self-reported health conditions and pesticide exposures on probability of follow-up in a prospective cohort study.

BACKGROUND

We investigated the potential for selection bias due to non-participation in the follow-up of a large prospective cohort study.

METHODS

Licensed pesticide applicators (52,395 private; 4,916 commercial) in the Agricultural Health Study provided demographic, health, and pesticide exposure information at enrollment (1993-1997) and in a 5-year follow-up telephone interview. Factors associated with non-participation in the follow-up were identified using multiple logistic regression. Potential for selection bias was evaluated by comparing exposure-disease associations between the entire cohort and the follow-up subset.

RESULTS

Sixty-six percent of private and 60% of commercial applicators completed the follow-up interview. Private and commercial applicators who did not complete the follow-up reported at enrollment younger age, less education, lower body mass index, poorer health behaviors but fewer health conditions, and lower pesticide use. Estimates of exposure-disease associations calculated with and without non-participants did not indicate strong selection bias.

CONCLUSIONS

Differences between non-participants and participants in the follow-up interview were generally small, and we did not find significant evidence of selection bias. However, the extent of bias may depend on the specific exposure and outcome under study.