Case control study statistics definition

They showed a statistically significant association in a large case—control study. For example, if cases are selected from a defined population such as a GP register, then controls should comprise a sample from the same GP register. In all states, women aged 65 years or older were randomly selected from the lists of the Centers for Medicare and Medicaid Services Social Security lists.

The investigator then determines whether cases and controls were exposed or not exposed to the risk factor. When conducted prospectively, or when nested in a prospective cohort study, it is straightforward to select controls from the population at risk.

Preferably controls are drawn from the same population as the cases.

Difference between cohort and case control study ppt

For this reason, retrospective investigations are often criticised. In a case-control study the prevalence of exposure to a potential risk factor s is compared between cases and controls. In other words, the "would" criterion doesn't work here, because anyone in the state with clinical hepatitis would end up in the surveillance system, but someone who lived far from the deli would have a much lower likelihood of having the exposure. Friend, Neighbor, Spouse, and Relative Controls: Occasionally investigators will ask cases to nominate controls who are in one of these categories, because they have similar characteristics, such as genotype, socioeconomic status, or environment, i. In order for the controls to be representative of the "population" that produced those cases, the controls should be selected by a similar mechanism, e. All efforts should be made to avoid sources of bias such as the loss of individuals to follow up during the study. See Figure 1 for a pictorial representation of a case-control study design. Case control studies are observational because no intervention is attempted and no attempt is made to alter the course of the disease. Table 1. Incident cases comprise cases newly diagnosed during a defined time period. The patients undergoing joint replacement were similar in age and they also were following the same referral pathways. Case-base sampling also known as "case-cohort" sampling : Controls are selected from the population at risk at the beginning of the follow-up period in the cohort study within which the case-control study was nested. Issues in the design of case-control studies Formulation of a clearly defined hypothesis As with all epidemiological investigations the beginning of a case-control study should begin with the formulation of a clearly defined hypothesis. It is helpful to remember that it seems natural that the population denominator includes people who develop the disease in a cohort study.

Herbst et al. Similarly, it would be inappropriate to use patients from the emergency room, because the selection of a hospital for an emergency is different than for cancer, and this difference might be related to the exposure of interest.

Types of case control studies

In addition, the temporal sequence of exposure and disease is easier to assess among incident cases. The exposure status of each is determined, but they are not followed into the future for further development of disease. Risk Set Sampling: In the nested case-control study a control would be selected from the population at risk at the point in time when a case was diagnosed. A summary of the pros and cons of case-controls are provided in Table 1. Obviously, this is a much more efficient design. Population based case control studies are generally more expensive and difficult to conduct. Come up with an answer to this question and then click on the icon to the left to reveal the answer. The case—control study is frequently contrasted with cohort studies , wherein exposed and unexposed subjects are observed until they develop an outcome of interest. The cluster of cases in young women was initially reported as a case series, but there were no strong hypotheses about the cause. Of all women contacted, 68 percent of cases and 42 percent of controls participated in the study.

An observational study is a study in which subjects are not randomized to the exposed or unexposed groups, rather the subjects are observed in order to determine both their exposure and their outcome status and the exposure status is thus not determined by the researcher.

Prospective studies usually have fewer potential sources of bias and confounding than retrospective studies.

case control study vs cohort study
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Introduction to study designs