5.2. Types of Observational Studies: Case-control studies

Retrospective studies, and in particular case-control studies, are susceptible to the effects of bias, which may be introduced during the collection of exposure and outcome data. Three kinds of bias are predominant and are briefly described in the following. Bias decreases the internal validity of the investigation and should be addressed and reduced in the study design as there are limited options for the control of bias during the analysis.

Recall bias: When information about exposure is collected by self-report, it is susceptible to recall bias. Recall bias occurs e.g., when a differential response between cases and controls occurs. The common scenario is when a participant with disease (case) will unconsciously recall and report an exposure with better clarity due to the disease experience.

Interviewer/observer bias Interviewer bias occurs when the interviewer asks leading questions or has an inconsistent interview approach between cases and controls or the interviewer’s knowledge of an individual's disease status is insufficient. A good study design will implement a standardised interview in a non-judgmental atmosphere with well-trained interviewers to reduce interviewer bias.

Selection bias: Selection bias is a particular problem in case-control studies, leading to non-comparability between cases and controls when cases (or controls) are included in, or excluded from a study because of some characteristic they show, related to exposure to the risk factor under evaluation. It may occur when those individuals selected as controls are unrepresentative of the population providing the cases. Selection bias may also be introduced when exposed cases are more likely to be selected than unexposed cases. The potential for selection bias may be minimized by selecting controls from more than one source, such as by using both hospital and neighbourhood controls.