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In essence, a case-control strategy was used, but it was conducted within the context of a prospective cohort study. This is referred to as a case-control study "nested" within a cohort study. Rothman states that one should look upon all case-control studies as being "nested" within a cohort.
In other words the cohort represents the source population that gave rise to the cases. With a case-control sampling strategy one simply takes a sample of the population in order to obtain an estimate of the exposure distribution within the population that gave rise to the cases. Obviously, this is a much more efficient design.
It is important to note that, unlike cohort studies, case-control studies do not follow subjects through time. Cases are enrolled at the time they develop disease and controls are enrolled at the same time. The exposure status of each is determined, but they are not followed into the future for further development of disease.
As with cohort studies, case-control studies can be prospective or retrospective. At the start of the study, all cases might have already occurred and then this would be a retrospective case-control study.
Alternatively, none of the cases might have already occurred, and new cases will be enrolled prospectively. Epidemiologists generally prefer the prospective approach because it has fewer biases, but it is more expensive and sometimes not possible. When conducted prospectively, or when nested in a prospective cohort study, it is straightforward to select controls from the population at risk.
However, in retrospective case-control studies, it can be difficult to select from the population at risk, and controls are then selected from those in the population who didn't develop disease.
Using only the non-diseased to select controls as opposed to the whole population means the denominator is not really a measure of disease frequency, but when the disease is rare, the odds ratio using the non-diseased will be very similar to the estimate obtained when the entire population is used to sample for controls.
This phenomenon is known as the rare-disease assumption. When case-control studies were first developed, most were conducted retrospectively, and it is sometimes assumed that the rare-disease assumption applies to all case-control studies. However, it actually only applies to those case-control studies in which controls are sampled only from the non-diseased rather than the whole population.
The difference between sampling from the whole population and only the non-diseased is that the whole population contains people both with and without the disease of interest. This means that a sampling strategy that uses the whole population as its source must allow for the fact that people who develop the disease of interest can be selected as controls.
Students often have a difficult time with this concept. It is helpful to remember that it seems natural that the population denominator includes people who develop the disease in a cohort study.Case series: Population Full epidemiological use of case-series data needs information on the population to permit calculation of rates Key to understanding the distribution of disease in populations and to the study of variations over time, between places and by population characteristics Case-series can provide the key to sound case control.
by Annette Gerritsen, Ph.D. Two designs commonly used in epidemiology are the cohort and case-control studies. Both study causal relationships between a risk factor and a disease.
Case-Control Studies for Outbreak Investigations Goals. Describe the basic steps of conducting a case-control study; Discuss how to select cases and controls; Discuss how to .
Design and Analysis of Case-Control Studies Kyoungmi Kim, Ph.D. Nov 9 & 16, This seminar is jointly supported by the following NIH-funded centers: A case-control study is preferred when the disease is rare because .
1 Daniel E. Ford, MD, MPH STUDY DESIGN CASE-CONTROL Daniel E. Ford, MD, MPH Vice Dean of Clinical Investigation Johns Hopkins School of Medicine Introduction to Clinical Research. This statement is applicable to cohort studies, case-control studies, and cross-sectional studies.
In fact, 18 of the checklist items are common to all three types of observational studies, and 4 items are specific to each of the 3 specific study designs.