Association of attrition with mortality: Findings from 11 waves over three decades of the Whitehall II study

BACKGROUND: Attrition, the loss of participants as a study progresses, is a considerable challenge in longitudinal studies. This study examined whether two forms of attrition, “withdrawal” (formal discontinued participation) and “non-response” (non-response among participants continuing in the study) have different associations with mortality, and whether these associations differed across time in a multi-wave longitudinal study. METHODS: Participants were 10 012 civil servants who participated at the baseline of the Whitehall II cohort study with 11 data waves over average follow-up of 28 years. We performed competing-risks analyses to estimate sub-distribution hazard ratios and 95% confidence intervals, and likelihood ratio tests to examine whether hazards differed between the two forms of attrition. We then applied linear regression to examine any trend of hazards against time. RESULTS: Attrition rate at data collections ranged between 13% and 34%. There were 495 deaths recorded from cardiovascular disease and 1367 deaths from other causes. Study participants lost due to attrition had 1.55 (95% confidence interval 1.26 to 1.89) and 1.56 (1.39 to 1.76) times higher hazard of cardiovascular and non-cardiovascular mortality than responders respectively. Hazards for withdrawal and non-response did not differ for either cardiovascular (p-value = 0.28) or non-cardiovascular mortality (p-value = 0.38). There was no linear trend in hazards over the 11 waves (cardiovascular mortality p-value = 0.11, non-cardiovascular mortality p-value = 0.61). CONCLUSION: Attrition can be a problem in longitudinal studies resulting in selection bias. Researchers should examine the possibility of selection bias and consider applying statistical approaches that minimise this bias.