Confounding bias and effect modification in epidemiologic research.

The medical literature contains many studies of the clinical effectiveness of diagnostic tests and therapeutic interventions. Common problems in experimental design occur that influence the usefulness of original research. Confounding bias and effect modification are two important factors that affect whether clinicians ought to apply the findings of clinical research to the care of their patients. Investigators should minimize confounding biases in their work. Effect modification should be described so that readers can decide which of their patients will benefit from a particular study. This article uses a number of clinical examples to help the clinician and investigator understand the influences of bias and effect modification on clinical research.