Bias in clinical epidemiological study designs.

Systematic error, or bias, is error that occurs in each measurement made and which has a direction, i.e., the measured value is always either greater or smaller than the true value. The presence of systematic error directly affects the internal validity of the study, and indirectly affects the external validity of the results obtained. In general, such error can be classified as selection bias, classification bias or confounding bias. It is essential to deal with possible bias in the research design phase, since only confounding bias can be controlled in the phase corresponding to analysis of the results.