2.1.3. Case-control studies

In contrast to cohort studies where participants are identified by exposure status, participants in case-control studies are identified by their disease or outcome status. Cases are participants who have developed the outcome of interest. Controls are subjects who do not have the outcome of interest and provide an estimate of the  frequency of exposure in the population at risk. In this retrospective study design, cases and controls are first identified. Subsequently, the exposure to the factor of interest is ascertained, for each case and control. Lastly, an odds ratio for the outcome of interest (in relation to exposure status) is calculated. Case-control studies are retrospective because they seek to determine previous exposure after the outcome has been established. Thus, they are subject to recall or information bias. Case-control studies are also subject to sampling bias because it is difficult to select controls which are (ideally) perfectly similar to cases, with the exception of outcome status. However, techniques such as matching controls to cases and stratified analysis can improve the precision of estimates from case-control studies.

Case-control studies are useful when attempting to isolate a cause or causes for an emerging disease condition. Most recently this approach was used in attempts to determine the factors contributing  to Colony Collapse Disorder (CCD) (vanEngelsdorp et al., 2009b, 2010; Dainat et al., 2012) Data analysis and interpretation