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Catalogue of bias: attrition bias

David Nunan1, Jeffrey Aronson2, Clare Bankhead1

  • 1Nuffield Department of Primary Care Health Sciences, Centre for Evidence-Based Medicine, University of Oxford, Oxford, UK.

BMJ Evidence-Based Medicine
|January 26, 2018
PubMed

Insights

Attrition bias, a form of selection bias, occurs when participants are systematically lost from studies. Removing studies with high attrition bias risk can alter research findings, highlighting the need for preventive strategies.

Area of Science:

  • Medical research methodology
  • Clinical trial design
  • Evidence-based medicine

Background:

  • Attrition bias is a critical type of selection bias in research.
  • It arises from systematic differences in participant loss between study groups.
  • This bias can distort study results, attributing effects to interventions when they are due to participant dropout.

Purpose of the Study:

  • To describe attrition bias and its impact on research.
  • To outline preventive measures for minimizing attrition bias.
  • To illustrate the effect of attrition bias on study conclusions.

Main Methods:

  • Literature review and analysis of bias within the Catalogue of Bias series.
  • Examination of existing research methodologies and their susceptibility to attrition bias.
  • Case study analysis of tranexamic acid trials and upper gastrointestinal bleeding.

Main Results:

  • Observed associations for mortality in tranexamic acid trials were no longer significant after excluding studies with high attrition bias.
  • This demonstrates how attrition bias can mask or create false associations.
  • Systematic differences in participant loss can be the true driver of observed effects, not the intervention.

Conclusions:

  • Attrition bias poses a significant threat to the validity of research findings.
  • Preventive strategies like over-recruitment are crucial.
  • Methods such as sampling weights and replenishment samples can mitigate the impact of existing attrition bias.

Keywords:
clinical trialsepidemiology

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