Over-matching might make it harder to identify enough controls. Though some controls have to be similar to cases in many respects, it is possible to over-match. Another key feature of a case-control study is that the exposure in both cases and controls should be measured equally. For instance, it would be unfair to compare patients with elective intraocular surgery to a group of controls with traumatic corneal lacerations. The matching criteria are the parameters (e.g., age, sex, and hospitalization time) used to establish how controls and cases should be similar. A disease’s definition may be based on many criteria at times hence, all aspects should be fully specified in the case definition.Ĭontrols that are comparable to the cases in a variety of ways should be chosen. The cases should be defined as precisely as feasible by the investigator. Example of a case-control study Selection of case related methods are risk (prospective), relative risk meta-analysis, risk difference meta-analysis and proportionsĬopyright © 2000-2022 StatsDirect Limited, all rights reserved.prone to the bias of change in methods over time.prone to attrition bias (compensate by using person-time methods).may uncover unanticipated associations with outcome.yields true incidence rates and relative risks.The following notes relate cohort to case-control studies: related methods are risk (retrospective), chi-square 2 by 2 test, Fisher's exact test, exact confidence interval for odds ratio, odds ratio meta-analysis and conditional logistic regression.Ĭohort studies are usually but not exclusively prospective, the opposite is true for case-control studies.controls are selected on the basis of not having the outcome.The following notes relate case-control to cohort studies: Prospective investigation is required to make precise estimates of either the incidence of an outcome or the relative risk of an outcome based on exposure.Ĭase-Control studies are usually but not exclusively retrospective, the opposite is true for cohort studies. You should take special care to avoid sources of bias and confounding in retrospective studies. In retrospective studies the odds ratio provides an estimate of relative risk. If the outcome of interest is uncommon, however, the size of prospective investigation required to estimate relative risk is often too large to be feasible. For this reason, retrospective investigations are often criticised. Most sources of error due to confounding and bias are more common in retrospective studies than in prospective studies. Many valuable case-control studies, such as Lane and Claypon's 1926 investigation of risk factors for breast cancer, were retrospective investigations. Prospective studies usually have fewer potential sources of bias and confounding than retrospective studies.Ī retrospective study looks backwards and examines exposures to suspected risk or protection factors in relation to an outcome that is established at the start of the study. All efforts should be made to avoid sources of bias such as the loss of individuals to follow up during the study. The outcome of interest should be common otherwise, the number of outcomes observed will be too small to be statistically meaningful (indistinguishable from those that may have arisen by chance). The study usually involves taking a cohort of subjects and watching them over a long period. A prospective study watches for outcomes, such as the development of a disease, during the study period and relates this to other factors such as suspected risk or protection factor(s).

Case control study