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 an average follow-up of 28 years. We performed competing-risks analyses to estimate sub-distribution HRs and 95% CIs, 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% CI 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.
- SOCIAL EPIDEMIOLOGY
- Social and life-course epidemiology
- Stochastic Porcess
- Epidemiological methods
- MEDICAL STATISTICS
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↵* ON and MJS are senior authors.
Contributors MA conceived the initial idea for the paper and elaborated this with all authors; MA and ON designed the study, and MA, ON and MJS undertook statistical analysis. MA, MK, AS, ON and MJS drafted the article. All authors contributed to the critical revision and the final approval of the version to be published. MA is guarantor. All authors had agreement to be accountable for the accuracy and integrity of the study. The corresponding author affirms that all authors meet authorship criteria recommended in the International Committee of Medical Journal Editors (ICMJE) Recommendations for the Conduct, Reporting, Editing and Publication of Scholarly Work in Medical Journals 2013. The corresponding author attests that all listed authors meet authorship criteria and that no others meeting the criteria have been omitted.
Funding The study was supported by the UK Medical Research Council (MRC, K013351; R024227), the US National Institute on Aging (NIA, R56AG056477; R01AG0564779) and the British Heart Foundation (32334). MA is funded by a PhD scholarship from the Foundation for Advanced Studies on International Development (FASID). MK is supported by the MRC (S011676, R024227), NIA (R01AG0564779), NordForsk, Academy of Finland (311492) and Helsinki Institute of Life Science. MJS is partly supported by the British Heart Foundation. The funders did not contribute to the study design, data collection, analysis, interpretation, drafting nor the decision to publish the study.
Competing interests All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf and declare: no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous 3 years; no other relationships or activities that could appear to have influenced the submitted work.
Patient consent for publication Not required.
Ethics approval It was not required since the study used secondary data. The Joint University College London/University College London Hospital Committees on the Ethics of Human Research has approved the Whitehall II study.
Data sharing statement Data of the Whitehall II study are available to the scientific community. Data sharing policy is available at https://www.ucl.ac.uk/epidemiology-health-care/research/epidemiology-and-public-health/research/whitehall-ii/data-sharing.
Provenance and peer review Not commissioned; externally peer reviewed.
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