Avoiding bias in observational studies: part 8 in a series of articles on evaluation of scientific publications.

BACKGROUND: Many questions in human health research can only be answered with observational studies. In contrast to controlled experiments or well-planned, experimental randomized clinical trials, observational studies are subject to a number of potential problems that may bias their results.

METHODS: Some of the more important problems affecting observational studies are described and illustrated by examples. Additional information is provided with reference to a selection of the literature.

RESULTS: Factors that may bias the results of observational studies can be broadly categorized as: selection bias resulting from the way study subjects are recruited or from differing rates of study participation depending on the subjects' cultural background, age, or socioeconomic status, information bias, measurement error, confounders, and further factors.

CONCLUSIONS: Observational studies make an important contribution to medical knowledge. The main methodological problems can be avoided by careful study planning. An understanding of the potential pitfalls is important in order to critically assess relevant publications.