SeriesCase-control studies: research in reverse
Section snippets
Basic case-control study design
Case-control designs might seem easy to understand, but many clinicians stumble over them. Because this type of study runs backwards by comparison with most other studies, it often confuses researchers and readers alike. In cohort studies, for example, study groups are defined by exposure. In case-control studies, however, study groups are defined by outcome (figure). To study the association between smoking and lung cancer, therefore, people with lung cancer are enrolled to form the case
Advantages and disadvantages
Epidemiologists often tout case-control studies as the most efficient design in terms of time, money, and effort. This recommendation makes sense when the incidence rate of an outcome is low, since in a cohort design the researchers would have to follow up many individuals to identify one with the outcome. Case-control studies are also efficient in the investigation of diseases that have a long latency period—eg, cancer—in which instance a cohort study would involve many years of follow-up
Case group
All the cases from a population could, theoretically, be included as participants in a case-control study. For practical reasons, however, only a sample is frequently studied.22 Investigators should, therefore, state how the sample was selected, providing a clear definition of the outcome being studied including, for example, clinical symptoms, laboratory results, and diagnostic methods used. Furthermore, researchers should detail eligibility criteria used for selection, such as age range and
Measurement of exposure information
Another difficulty in case-control studies involves the measurement of exposure information. Participants, both cases and controls, might inaccurately remember past exposures, especially those that happened a long time ago. Furthermore, cases often remember exposures to putative risk factors differently than controls. This differential recall (recall bias) causes information bias.25
In the study of breast cancer and oral contraceptive use,27 for example, investigators asked participants about
Control for confounding
Case-control studies need to address confounding bias.21, 22, 29 This type of bias can be dealt with in the design phase by restriction or matching, but researchers generally prefer to handle it in the analysis phase with analytical techniques such as logistic regression or stratification with Mantel-Haenszel approaches.21, 22, 25 If this second approach is used, investigators should plan carefully in advance what potentially confounding variables to obtain data for; irrespective of the
Conclusion
Case-control studies that are well designed and carefully done can provide useful and reliable results. Investigators must, however, devote meticulous attention to the selection of control groups and to measurement of exposure information. Awareness of these key elements should help readers to identify the strengths and weaknesses of a properly reported study. Accurate and thorough description of methods by investigators will result in reader confidence in their results.
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