Original article
A high response is not essential to prevent selection bias: Results from the Leiden 85-plus study

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Abstract

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 particpated, the 88 subjects who entered the sudy 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.

Introduction

A highly representative sample of participants is no longer considered essential for generalizability in etiologic studies that report risk estimates rather than prevalence estimates 1, 2, 3. Even a minimum of 80% response in follow-up studies is debatable [4]. Generalizability depends on the ability to abstract universal scientific hypotheses or theories from a set of observations and not only from the statistical framework of these observations 5, 6. However, many studies in the elderly have a public health goal in addition to more scientific etiologic goals. In such community surveys, generating estimates that can be extrapolated to the general population, representativeness is still very important. Furthermore, it is essential to include frail elderly subjects in a study to investigate the determinants of and causal relations with chronic conditions. Refusal to participate due to ill health would surely invalidate results on the impact of chronic conditions in an elderly population [7].

A high response rate increases the validity of community-based studies, because a low response rate might lead to selection bias [8]. The success of the response depends to a great extent on the way eligible subjects are approached. A high response can be achieved by interviewing and examining elderly subjects in their homes, because frail elderly subjects are less inclined to visit a study site [9]. Other effective strategies to optimize response rates are notification in advance by mail, involvement of expert researchers, and the prospect of a small gift [10]. Another possibility to increase response is to approach eligible subjects who initially declined or did not respond at all 11, 12. Using these strategies surveys among the elderly have been conducted resulting in response rates between 60 and 90% 1, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23. Differences in characteristics and associations between the sample of participants and the source population, however, frequently remain unknown.

In the Leiden 85-plus Study, a research nurse visited all subjects who did not participate directly after the first approach by telephone. Through this additional effort more subjects were drawn into the study. Moreover, the nurse asked a few questions to those who refused to participate to get an impression of their health and well-being. In this way we collected data from the whole source population. This provided an excellent opportunity to test the hypothesis that the additional effort to increase the response rate had diminished selection bias.

Section snippets

Methods

The Leiden 85-plus Study is a series of gerontologic surveys of the population of the oldest old living in the town of Leiden, The Netherlands. The first survey started in 1986. The present survey is a community-based follow-up study in a delineated cohort of 85 year olds. Special topics within the Leiden 85-plus Study are atherosclerosis, cognitive function, chronic diseases, disabilities, and well-being.

Results

Between 1 September 1997 and 1 September 1999, 705 inhabitants of Leiden reached the age of 85. Fourteen inhabitants died before they could be enrolled in the study, and thus 691 subjects were eligible to participate in the study. A total of 511 subjects, the direct sample, participated directly after invitation by phone, resulting in a response rate of 74%. After the additional recruitment stage another 88 subjects were included after being personally approached by our research nurse. As a

Discussion

The design of our study in which vitually all subjects from the source population were visited at their place of residence, gave us the unique opportunity to compare characteristics of subjects from different samples of participants with all the subjects from the source population, including the nonparticipants. We tested the hypothesis that the additional effort to increase the response rate would diminish selection bias. We found that the direct sample with a response rate of 74% was

Acknowledgements

This study was partly funded by the Dutch Ministry of Health, Welfare and Sport. We would especially like to thank our expert nurse, Inge Mooijekind, for her enthusiasm and dedication in visiting all the subjects. Dr. Jan Vandenbroucke is acknowledged for his critical comments.

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