Introduction There are well-established risk factors, such as lower education, for attrition of study participants. Consequently, the representativeness of the cohort in a longitudinal study may deteriorate over time. Death is a common form of attrition in cohort studies of older people. The aim of this paper is to examine the effects of death and other forms of attrition on risk factor prevalence in the study cohort and the target population over time.
Methods Differential associations between a risk factor and death and non-death attrition are considered under various hypothetical conditions and data from the Australian Longitudinal Study on Women's Health (ALSWH) and the Australian Censuses and National Health Surveys are used to illustrate the evolution of bias over 12 years.
Results Between 1996 and 2008, 28.4% of ALSWH participants born in 1921–1926 died, 16.5% withdrew and 10.4% were lost to follow-up. Hypothetical scenarios illustrate how death and other attrition can affect changes in bias over time. For this cohort there were differential associations with various risk factors, for example, being born in a non-English speaking country was associated with non-death attrition but not death whereas being underweight was associated with death but not other forms of attrition.
Conclusions Deaths occur in both the target population and study cohort, while other forms of attrition occur only in the study cohort. Therefore non-death attrition may cause greater bias than death in longitudinal studies. However although more than a quarter of the oldest participants in the ALSWH died, differences from the national population changed only slightly.