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Sampling strategies for selecting general population comparison cohorts.

Uffe Heide-Jørgensen1, Kasper Adelborg1, Johnny Kahlert1

  • 1Department of Clinical Epidemiology, Aarhus University Hospital, Aarhus, Denmark, uhj@clin.au.dk.

Clinical Epidemiology
|October 13, 2018
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Summary

Sampling comparison cohorts without replacement in random order can introduce bias, particularly in older individuals. Sampling with replacement or 1:1 ratios avoids this bias in matched cohort studies.

Keywords:
matched cohort studyobservational studypopulation-based registrysurvival analysis

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Area of Science:

  • Epidemiology
  • Biostatistics

Background:

  • Population-based registries enable comparison cohort sampling for disease understanding.
  • Sampling without replacement in random order may cause immortal time bias.

Purpose of the Study:

  • Compare sampling strategies for comparison cohorts in matched cohort studies.
  • Evaluate impact on ischemic stroke and mortality risks.
  • Assess bias introduced by different sampling methods.

Main Methods:

  • Constructed index cohorts from the Danish general population.
  • Applied sampling strategies (without replacement, with replacement, chronological order) at 5:1 or 1:1 ratios.
  • Compared outcome risks and sampled cohorts for a heart failure cohort.

Main Results:

  • Sampling 5:1 without replacement in random order increased outcome risks, especially for individuals >50 years.
  • Chronological sampling failed to yield sufficient comparators in older cohorts, showing lower mortality.
  • 1:1 sampling and sampling with replacement showed no systematic bias.

Conclusions:

  • Sampling 5:1 without replacement introduces bias in older, high-risk index cohorts.
  • Sampling with replacement or 1:1 ratio effectively mitigates bias in comparison cohort selection.