Original article
Psychological attributes and changes in disability among low-functioning older persons: Does attrition affect the outcomes?

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Abstract

The impact of attrition was studied in a two-wave panel study on the associations between psychological attributes (mastery, self-efficacy expectancies) and disability increase among low-functioning older persons (n = 753, 76.4% participated at the follow-up 2 years later). The results showed that: (1) attrition at follow-up was clearly associated with age and level of disability at baseline. Older persons and those with higher levels of disability did not participate in the follow-up; and (2) the strength of the association between the selected psychological attributes and the increase in disability did not substantially alter when (a) missing follow-up scores of disability were replaced by several values (e.g., lower, equal or higher compared to those who actually participated at the follow-up), or (b) associations were computed excluding those participants at follow-up with highest increase in disability. We conclude that attrition may affect the descriptive outcomes of aging studies, particularly when such studies focus on health and function, but that attrition not always seems to be a serious problem when associations between variables are the focus of study.

Introduction

The effects of attrition of study participants from one wave of data collection to the next are of concern to researchers conducting panel studies. Particularly, research in the field of clinical medicine, geriatrics, and gerontology is often complicated by the fact that nonresponse is substantially associated with morbidity and age. Specific to the study of the elderly is the burden of decline in all areas, which is much greater than in younger populations. This has consequences for the participation rate that should be achieved 1, 2. The question rises whether the outcome of studies in this area may be generalized to the population where the study sample comes from. Particularly in longitudinal or follow-up studies where attrition is related to the entrance in the study as well as to participation at follow-up, generalizability may become a serious problem. Several studies have been conducted on the determinants of attrition in aging studies. Powell et al. [3], for example, showed that impaired physical health and a general disengagement from social and personal activities were associated with attrition in older persons. Slymen et al. [4] found that attrition was related to older age, lower income, and poorer health. However, much less attention has been paid to an examination of the impact of attrition on specific research outcomes. In the present paper, we seek to examine the effect of attrition at follow-up on the outcomes of a study in low-functioning elderly persons. More specifically, we study whether an empirical verification of a research hypothesis on the impact of two psychological attributes on changes in disability over 2 years' time is affected by attrition. The results of this study concerning the impact of psychological attributes on the trajectories of disability have been published previously [5]. The researchers hypothesized that high levels of both mastery and self-efficacy expectancies slow down the progression of disability. Their findings confirmed this hypothesis. To test their hypothesis, the researchers selected a low-functioning sample of 753 older persons, as changes in functional ability within a 2-year interval were particularly expected in moderately disabled elders. Five hundred seventy-five persons participated in a follow-up, 2 years later. Attrition was due to mortality (n = 58), poor physical and/or mental health (n = 66), and refusal (n = 54); the 178 nonparticipants at follow-up were excluded from the original paper. In the present paper, we particularly focus on the impact of the attrition of these 178 persons (23.6%) on the outcomes of the study.

Section snippets

Sample

The data were obtained from a subsample (n = 753) of the baseline participants (n = 5,279) in the Groningen Longitudinal Aging Study (GLAS) in 1993. This is a population-based prospective follow-up study of the determinants of health-related quality of life of elderly people, with special emphasis on physical and social disability and well being 6, 7, 8, 9, 10. The primary objective of GLAS is to identify the psychosocial factors that influence the trajectory of the quality of life, either

Results

Table 1 shows the descriptive baseline statistics for those who continued to participate (n = 575), for those who did not participate at follow-up due to mortality (n = 58), poor physical and/or mental health (n = 66) and refusal (n = 54), and for the total baseline sample (n = 753). The results show that continuing participants were younger than those who did not participate due to mortality or health problems. The mean age of the refusers was similar to the mean age of the continuing

Conclusion and discussion

The main objective of the present panel study was to analyze whether research outcomes from a previous study [5] were affected by attrition at follow-up. The researchers hypothesized and reported that high levels of mastery and self-efficacy expectancies slow down the progression of disability in low-functioning older persons. What can be concluded from the results of the present study? First, attrition at follow-up is clearly related to age and levels of disability at baseline. Particularly

Acknowledgements

This research is part of the Groningen Aging study (GLAS). GLAS is conducted by the Northern Centre for Healthcare Research (NCH) and various Departments of the University of Groningen in The Netherlands. The primary departments involved are Health Sciences, Family Medicine, Psychiatry, Sociology (ICS) and Human Movement Sciences. GLAS and its substudies are financially supported by the Dutch government (through NESTOR), the University of Groningen, the Faculty of Medical Sciences, the Dutch

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