Case control studies in epidemiology
Given how uncommon the outcome was, even a large prospective study would have been unlikely to have more than one or two cases, even after years of follow-up. The fact that the analysis is retrospective, allows rare diseases or diseases with long latency periods to be investigated.
How to select controls in case control study
Nevertheless, as case-controls are retrospective, they are more prone to bias. For this reason, retrospective investigations are often criticised. Analysis yields an odds ratio OR that reflects the relative probabilities of exposure in the two populations. Initially, they will often use a somewhat broad definition in order to identify potential cases. They are comparatively quick, inexpensive, and easy. Cohort studies can assess a range of outcomes allowing an exposure to be rigorously assessed for its impact in developing disease. Because of their efficiency, they may also be ideal for preliminary investigation of a suspected risk factor for a common condition; conclusions may be used to justify a more costly and time-consuming longitudinal study later. An important technique for adding power to a study is to enroll more than one control for every case. The problem is that the controls were selected by a different mechanism than the cases, AND the selection mechanism may have tended to select individuals of different socioeconomic status, since women who were at home may have been somewhat more likely to be unemployed. TEM might cause corneal ulcers but it is also possible that the presence of a corneal ulcer leads some people to use TEM. The case-control design also is the most practical option for studying exposure related to rare diseases. Case Reports Definition A study that compares patients who have a disease or outcome of interest cases with patients who do not have the disease or outcome controls , and looks back retrospectively to compare how frequently the exposure to a risk factor is present in each group to determine the relationship between the risk factor and the disease. In our example, controls could be defined as patients who underwent elective intraocular surgery during the same period of time. However, for the time being the key things to remember are that: The sampling strategy for a case-control study is very different from that of cohort studies, despite the fact that both have the goal of estimating the magnitude of association between the exposure and the outcome.
After World War II, DES started being prescribed for women who were having troubles with a pregnancy -- if there were signs suggesting the possibility of a miscarriage, DES was frequently prescribed. They are advantageous when studying dynamic populations in which follow-up is difficult.
Then, look back in time to learn which subjects in each group had the exposure scomparing the frequency of the exposure in the case group to the control group.
As a measure of the strength of the association between an exposure and the outcome, case-control studies yield the odds ratio. The exposure status of each is determined, but they are not followed into the future for further development of disease.
Resource text Case-control studies start with the identification of a group of cases individuals with a particular health outcome in a given population and a group of controls individuals without the health outcome to be included in the study.
They are inefficient for rare exposures.
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