Calculation of odds ratio different for matched and unmatched case control studies
In all three situations, cost and feasibility become an important concern. A case-control design offers an alternative that is much more efficient.
The goal of a case-control study is the same as that of cohort studies, i. However, case-control studies employ a different sampling strategy that gives them greater efficiency. As with a cohort study, a case-control study attempts to identify all people who have developed the disease of interest in the defined population.
This is not because they are inherently more important to estimating an association, but because they are almost always rarer than non-diseased individuals, and one of the requirements of accurate estimation of the association is that there are reasonable numbers of people in both the numerators cases and denominators people or person-time in the measures of disease frequency for both exposed and reference groups.
However, because most of the denominator is made up of people who do not develop disease, the case-control design avoids the need to collect information on the entire population by selecting a sample of the underlying population. The source population is the population that gives rise to the cases included in the study.
If a cohort study were undertaken, we would define the exposed and unexposed cohorts or several cohorts and from these populations obtain denominators for the incidence rates or risks that would be calculated for each cohort. We would then identify the number of cases occurring in each cohort and calculate the risk or incidence rate for each. In a case-control study the same cases are identified and classified as to whether they belong to the exposed or unexposed cohort.
Instead of obtaining the denominators for the rates or risks, however, a control group is sampled from the entire source population that gives rise to the cases. Individuals in the control group are then classified into exposed and unexposed categories. The purpose of the control group is to determine the relative size of the exposed and unexposed components of the source population.
To illustrate this consider the following hypothetical scenario in which the source population is Plymouth County in Massachusetts, which has a total population of 6, hypothetical.
Chapter 9. Case–Control Studies | Medical Epidemiology, 4e | AccessPharmacy | McGraw-Hill Medical
Thirteen people in the county have been diagnosed with an unusual disease and seven of them have a particular exposure that is suspected of being an important contributing factor. The chief problem here is that the disease is quite rare. If I somehow had exposure and outcome information on all of the subjects in the source population and looked at the association using a cohort design, it might look like this:.
Consequently, the risk ratio would be 0. This is a strong association. In this hypothetical example, I had data on all 6, people in the source population, and I could compute the probability of disease i. The problem , of course, is that I usually don't have the resources to get the data on all subjects in the population. An alternative approach would be to use surveillance databases or administrative databases to find most or all 13 of the cases in the source population and determine their exposure status.
However, instead of enrolling all of the other 5, residents, suppose I were to just take a sample of the non-diseased population. The data might look something like this:. With this sampling approach I can no longer compute the probability of disease in each exposure group, because I no longer have the denominators in the last column. In other words, I don't know the exposure distribution for the entire source population.
However, the small control sample of non-diseased subjects gives me a way to estimate the exposure distribution in the source population.
So, I can't compute the probability of disease in each exposure group, but I can compute the odds of disease in each group. We will consider odds ratios and case-control studies in much greater depth in a later module. What is MyAccess? Otherwise it is hidden from view. Forgot Username? About MyAccess If your institution subscribes to this resource, and you don't have a MyAccess Profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus.
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