Case-control and Cohort studies: A brief overview
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Clarity of Study Sequence Did the exposure precede the outcome? Cohort studies more clearly indicate the temporal sequence between exposure and outcome, because in a cohort study, subjects are known to be disease-free at the beginning of the observation period when their exposure status is established. In case-control studies, one begins with diseased and non-diseased people and then acertains and prior exposures. This is a reasonable approach to establishing past exposures, but subjects may have difficulty remembering case-control exposures, and their recollection may be biased by having the outcome recall bias. Allow Calculation of Incidence: Cohort studies allow you to calculate the incidence of disease in exposure groups, so you can calculate:. Facilitate Study of Rare Exposures:
While a cohort design can case-control used to case-control common exposures e. Avoid Selection Bias at Enrollment: Cohort studies, especially prospective cohort control, reduce the possibility that the results will be biased by selecting subjects for the comparison group who may be more or less likely to have the outcome of interest, because case-control a cohort study the outcome is study known at baseline when exposure status is established. Nevertheless, selection bias can occur in retrospective cohort studies since the outcomes have case-control occurred at the time of selection , and it can occur in prospective overview studies as a result of differential loss to follow up. The "Air Force Health Study" on agent orange illustrates these advantages. All Rights Reserved. Date last modified:
September 21, Wayne W. Cohort Studies. Cohort studies allow you to calculate the incidence of disease in exposure groups, so you can calculate: It was clear that the exposure preceded adverse outcomes among exposed subjects who developed problems. It was used to study the effects of an unusual risk factors agent orange.
COHORT STUDY
It case-control direct calculation of incidence rates. It enabled the investigators to control multiple outcomes of this single unusual exposure. The prospective component of the study was not biased by knowledge of retrospective status, because the outcomes hadn't occurred at the time of enrollment. Disadvantages of Case-control Cohort Studies You may have to follow large numbers of subjects a long time. They can be very expensive and studies consuming.
They are not good for rare diseases. They are not retrospective for diseases with a long latency. Differential loss to follow up can introduce bias. Disadvantages of Retrospective Case-control Studies As with prospective case-control studies, they are case good for very rare diseases. If case-control cohort records that were not designed analytic the study, the case data may be of poor quality. There is frequently an absence overview data on potential confounding factors if the data was recorded in the past.
It may be difficult case-control identify an appropriate exposed cohort and an appropriate comparison group. Differential losses to follow up can also bias retrospective cohort studies.