Instructor Resource Chapter 12 Copyright © Scott B. Patten, 2015. Permission granted for classroom use with Epidemiology for Canadian Students: Principles, Methods & Critical Appraisal (Edmonton: Brush Education Inc. www.brusheducation.ca).

Chapter 12. Differential and nondifferential misclassification bias in analytical studies

Objectives Distinguish differential and nondifferential misclassification bias. Explain the effect of differential and nondifferential misclassification bias on the interpretation of epidemiological conclusions. Describe blinded outcome assessment.

Bias in review Bias comes from study-design defects. Bias cannot be corrected by increasing the sample size. Bias can be prevented, but often not corrected after the fact.

Differential misclassification bias This is systematic error that occurs due to misclassification of exposure or disease in a way that differs depending on the other axis of the classic 2 x 2 table. Recall bias in case-control studies is an example of this kind of bias. When misclassification is differential, there are no simple rules about the direction or magnitude of bias. Each case needs to be thought through individually.

Nondifferential misclassification bias This is systematic error that occurs due to classification errors that do not differ across the categories on the other axis of the 2 x 2 table. For example, misclassification of exposure that does not depend on disease status is nondifferential. In a case-control study, this would mean that the frequency with which errors in classification of exposure status does not differ depending on case versus control status. This type of bias is in the direction of the null (closer to 1 for an odds ratio, closer to 0 for a prevalence difference).

Addressing misclassification bias in critical appraisal Consider the quality of a study’s measurement strategies. Look for study-design procedures protecting against differential misclassification—for example, whether raters of exposure are “blind” to disease status in a case-control study.

Addressing misclassification bias in critical appraisal The implications of bias sometimes depend on its direction. A weak (or no-effect) result may not be a credible finding in a study that is biased toward the null by nondifferential misclassification bias. A study reporting a strong effect may be even more impressive in the face of such bias.

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