Non-response bias in a study of cardiovascular diseases, functional status and self-rated health among elderly men - United Kingdom

Age and Ageing, Jan, 1998 by Nancy Hoeymans, Edith J.M. Feskens, Geertrudis A.M. van den Bos, Daan Kromhout

Introduction

In elderly people, cardiovascular diseases are an important cause of diminished functional status and well-being [1-5]. However, studies on health outcomes of cardiovascular diseases may be biased due to selective non-response or drop-out. Many studies in elderly populations have reported that the health status of non-respondents is less than that of respondents [6-10], although examples of the opposite are also found [11]. Furthermore, follow-up studies have observed higher morbidity and mortality rates among non-respondents than among respondents [12-14]. Non-response may lead to bias not only in prevalence estimates of diseases and adverse health outcomes, but also in the associations between diseases and health outcomes [9, 13, 15, 16].

The Zutphen Elderly Study is a longitudinal study on life-style, chronic diseases and health and was started in 1985. A non-response survey was carried out during the follow-up survey in 1993 to quantify possible bias in measures of health status due to drop-out. The aim of our study was to investigate to what extent differences in health status between respondents and drop-outs affected the association between myocardial infarction and stroke on one hand and functional status and self-rated health on the other.

Methods

Study population

The Zutphen Elderly Study [17] was started in 1985 as a continuation of the Zutphen Study [18], a longitudinal population-based cardiovascular health study among men born between 1900 and 1920 and living in the town of Zutphen. In 1985, all survivors of the original cohort and a random sample of all other men in the same age range living in Zutphen were recruited. This resulted in a target population of 1266 men, of whom 939 (74%) participated. This group formed the cohort of the Zutphen Elderly Study. For the follow-up survey of 1993, all 548 survivors of this cohort were contacted. They received a letter in which the study was explained and a response note in which they could indicate whether they were willing to participate. A reminder and a phone contact followed this initial letter in order to reach as many men as possible. In Spring 1993, the 390 men (71%) who indicated they were willing to participate received a questionnaire by mail and were visited 1 week later by one of our research assistants to check the questionnaire for inconsistencies or missing items and to carry out a test for cognitive and one for physical function. The questionnaire could also be completed by a relative or caregiver.

In June 1993 those who indicated they did not want to participate or who had not responded (n = 158) received a very short questionnaire which they or a relative or caregiver were asked to complete and send back. In the accompanying letter it was explained that it was important to have some information from nonparticipants. When no reply was received after 2 weeks, non-respondents were interviewed by telephone or visited at home.

Non-respondents who did not participate in the nonresponse survey (n = 50) did not differ appreciably from participants in the non-response survey (n = 108) as regards age, socio-economic status and baseline health status. Regarding current health status, the prevalence of myocardial infarction and stroke was lower among these non-respondents than among the participating non-respondents (myocardial infarction 14%, stroke 4%). No information on functional status and self-rated health was available for this group. In this report the term `non-respondent' refers to participants in the 1993 non-response survey. Complete data on cardiovascular diseases, functional status and self-rated health were available for 381 respondents and 99 non-respondents.

Measurements

Questions on marital status, history of cardiovascular diseases, functional status and self-rated health were identical in the survey and in the non-response questionnaire. Data on socio-economic status were based on the 1985 survey. Socio-economic status was recorded by life-long occupation in four levels: professionals, managers and teachers, small-business owners, non-manual workers and manual workers. Marital status was recorded in four categories: married, never married, divorced and widowed.

Information on the prevalence of myocardial infarction and stroke was obtained from the (non-response) questionnaire and verified with hospital discharge data and written information from general practitioners. For definite myocardial infarction the final diagnosis was based on whether two of the following three criteria were met: a specified medical history, i.e. severe chest pain lasting for more than 20 min and not disappearing in rest, characteristic electrocardiogram changes and specific enzyme elevations. Stroke was defined as a sudden onset of neurological paralysis lasting longer than 24 h.

Functional status was measured as disabilities in daily routine activities. The questionnaire we used was adapted from the 11 countries study [19] and described in detail in a previous publication [20]. In short, the questionnaire consisted of 13 items each mentioning one basic activity of daily living (BADL), mobility or instrumental activity of daily living (IADL) item. Participants who reported that they needed help with at least one of the following activities: feeding oneself, getting in and out of bed, using the lavatory, dressing and undressing, washing and bathing oneself and walking between rooms were classified as disabled in BADL. Respondents who stated that they needed help with moving outdoors, using stairs, walking at least 400 m or carrying a heavy object for 100 m were classified as being disabled in mobility. IADL disability was not taken into account in this study.