THE EFFECT OF FOLLOW-UP ON LIMITING NONPARTICIPATION BIAS IN GENETIC EPIDEMIOLOGIC INVESTIGATIONS

The use of a comprehensive follow-up strategy to limit non-participati on bias was evaluated in a population-based case-control study of orof acial clefts. Birth parents were requested to provide exposure data, a nd index children and parents were asked to provide blood specimens. F ollow-up included telephone or postal reminders every two weeks for up to three months. Consent to participate was received from 281 (76.6%) case mothers and 246 (72.4%) case fathers. The corresponding totals f or controls were 279 (54.7%) and 245 (49.8%). Evaluation of participat ion rates by intensity of follow-up showed that 23% of case and 18% of control families consented without reminders (first stage); 81% of ca ses and 83% of controls agreed following one or two reminders (second stage); and the remainder of participants consented following three or more reminders (final stage). Cumulative distributions of sociodemogr aphic characteristics differed little between second and final stage p articipants. Odds ratios for maternal multivitamin use were similar be tween second and final stage participants, whereas those for maternal and paternal smoking tended to decline. Although follow-up measures we re necessary to enroll most families, use of more than two reminders d id not appear to increase the representativeness of the sample; howeve r, termination of recruitment after only two reminders would have led to different conclusions. Future studies require data collection proto cols that encourage participation from all population subgroups, and o ne alternative is presented.