The exposure status of each is determined, but they are not followed into the future for further development of disease.Īs with cohort studies, case-control studies can be prospective or retrospective. Cases are enrolled at the time they develop disease and controls are enrolled at the same time. It is important to note that, unlike cohort studies, case-control studies do not follow subjects through time. Obviously, this is a much more efficient design. 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. In other words the cohort represents the source population that gave rise to the cases. Rothman states that one should look upon all case-control studies as being "nested" within a cohort. This is referred to as a case-control study "nested" within a cohort study. In essence, a case-control strategy was used, but it was conducted within the context of a prospective cohort study. With this approach a similar estimate of risk was obtained after analyzing blood samples from only a small sample of the entire population at a fraction of the cost with hardly any loss in precision. Odds of Exposure: 360/1079 in the cases versus 432/2,446 in the non-diseased controls. If one were to analyze the blood samples of 2,878 of the non-diseased women (twice as many as the number of cases), one would obtain results that would look something like those in the next table. One could analyze all of the blood samples from women who had developed breast cancer, but only a sample of the whole cohort in order to estimate the exposure distribution in the population that produced the cases. There is, however, another more efficient alternative, i.e., to use a case-control sampling strategy. While 1,439 breast cancers is a disturbing number, it is only 1.6% of the entire cohort, so the outcome is relatively rare, and it is costing a lot of money to analyze the blood specimens obtained from all of the non-diseased women. The problem is that this would have cost almost $1.8 million, and the investigators did not have the funding to do this. If they had been able to afford analyzing all of the baseline blood specimens in order to categorize the women as having had DDT exposure or not, they would have found a risk ratio = 1.87 ( 95% confidence interva l: 1.66-2.10). Table of Breast Cancer Occurrence Among Women With or Without DDT Exposure If the investigators could have analyzed all 90,000 samples this is what they would have found the results in the table below. The problem is that there are almost 90,000 women and it would cost $20 to analyze each of the blood samples. Since they froze blood samples at baseline, they have the option of analyzing all of the blood samples in order to ascertain exposure to DDT at the beginning of the study before any cancers occurred. Eight years have passed since the beginning of the study, and 1.439 women in the cohort have developed breast cancer. The women are then followed, and, after about eight years, the investigators want to test the hypothesis that past exposure to pesticides such as DDT is a risk factor for breast cancer. After enrollment, the women provide baseline information on a host of exposures, and they also provide baseline blood and urine samples that are frozen for possible future use. Suppose a prospective cohort study were conducted among almost 90,000 women for the purpose of studying the determinants of cancer and cardiovascular disease.
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