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Ecological bias, confounding, and effect modification.

S Greenland1, H Morgenstern

  • 1Division of Epidemiology, UCLA School of Public Health 90024.

International Journal of Epidemiology
|March 1, 1989
PubMed
Summary
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Ecological bias can occur due to effect modification, not just confounding. Standardizing or controlling for variables may not remove this bias in ecological studies.

Area of Science:

  • Epidemiology
  • Biostatistics
  • Public Health

Background:

  • Ecological bias is often linked to confounding by group variables or associated risk factors.
  • Understanding the precise conditions for ecological bias is crucial for accurate population-level health research.

Purpose of the Study:

  • To demonstrate that effect modification, not solely confounding, can cause ecological bias.
  • To broaden the understanding of conditions leading to ecological bias beyond traditional confounding.
  • To assess the efficacy of standardization and ecological control in mitigating ecological bias.

Main Methods:

  • Theoretical analysis of bias in ecological studies.
  • Examination of the roles of confounding and effect modification.
  • Evaluation of covariate associations at group and individual levels.

Related Experiment Videos

Main Results:

  • Ecological bias can arise from effect modification, independent of whether the group variable is a confounder.
  • Extraneous risk factors can induce ecological bias even without individual-level association with the study variable.
  • Standardization and ecological control methods are often insufficient to eliminate ecological bias.

Conclusions:

  • The conditions for ecological bias are broader than those for individual-level confounding.
  • Effect modification is a significant, often overlooked, driver of ecological bias.
  • Standardization and ecological control are generally inadequate for removing bias stemming from effect modification.