All study designs have an inherent nature of incorporating errors in the various stages of implementation of a study.The major limitations that arise while deriving inferences in epidemiological study designs are chance, bias and confounding which if unidentified results in invalid findings and distortion of the final estimates.Bias is a systematic, non- random error, foreseen in all epidemiological study designs. It has to be avoided at all phases i.e. design, conduct, analysis or during the reporting phase of study design. A mistake nestimate of an exposure’s effect on the risk of disease occurs, unless avoided. 

Keywords: Bias; Epidemiology; Error; Review; Systematic Error.