RT Journal Article SR Electronic T1 Bias JF Journal of Epidemiology and Community Health JO J Epidemiol Community Health FD BMJ Publishing Group Ltd SP 635 OP 641 DO 10.1136/jech.2003.008466 VO 58 IS 8 A1 Delgado-Rodríguez, M A1 Llorca, J YR 2004 UL http://jech.bmj.com/content/58/8/635.abstract AB The concept of bias is the lack of internal validity or incorrect assessment of the association between an exposure and an effect in the target population in which the statistic estimated has an expectation that does not equal the true value. Biases can be classified by the research stage in which they occur or by the direction of change in a estimate. The most important biases are those produced in the definition and selection of the study population, data collection, and the association between different determinants of an effect in the population. A definition of the most common biases occurring in these stages is given.