A high response is not essential to prevent selection bias

We tested the hypothesis that an additional effort to increase the response rate would diminish selection bias in a community-based cohort study. In the Leiden 85-plus Study, all subjects of the town of Leiden who had reached their 85th birthday were informed of the study by mail and then asked to participate by telephone. In an additional recruitment stage, those subjects who did not participate directly were visited and personally asked to participate. When these subjects refused, some nonresponse questions were asked. In this way we collected data on the whole source population. Of 691 eligible elderly subjects, 511 subjects (74%) participated directly. Of those who did not participate directly, 88 subjects participated after the additional effort. The response rate increased from 74% to 87%. Compared to the 511 subjects who directly participated, the 88 subjects who entered the study after the additional effort had poorer health and lower survival. The subjects who refused were more healthy and had poorer mood. The direct sample did not differ from the source population with respect to socio-demographics, health, and mortality. In conclusion, we showed that given a moderately high direct response the additional effort was effective in increasing the response rate, but was also selective and was not necessary to prevent selection bias.