Sources of Bias

This chapter provides a more comprehensive summary of the various sources of bias. It defines a source of bias to be any aspect of a comparative study that results in a discrepancy between the empirical effect generated by the study and the causal effect of interest. The chapter considers the real-world conditions that can affect a study and potentially give rise to bias. These sources of bias are described informally in many epidemiological and social science publications, often in the context of particular studies or areas of research. The chapter also provides twenty general sources of bias that can be classified into 5 broad categories such as sampling of subjects, assignment to exposure states, adherence to nominal exposure states, exposure ascertainment and outcome measurement. Controlled Vocabulary Terms empirical process; outcomes; sampling; selection bias