Cohort Study

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Submitted By joi411
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This chapter explains why and when epidemiologists prefer one type of study over another and describes strengths and weaknesses of each approach.
To begin an epidemiologic study, we decide what to study.
For this discussion, let's say we want to study prenatal exposure to electric and magnetic fields and the effect on a baby's birthweight. We look at the existing literature on birthweight to assess current knowledge and data. It is important to know if others have conducted similar studies in case they have uncovered specific design limitations or useful results, and this information is helpful in understanding the context of one's own study.
We believe that known risks include prematurity, poor prenatal care, low socioeconomic status, non-white ethnicity, large size of the mother, younger or older mothers, smoking, alcohol consumption and a host of other factors. Electric and magnetic field exposures are not known risk factors but have not been studied extensively. Therefore we wish to study them.

Cohort Study
The "What will happen to me?" study follows a group of healthy people with different levels of exposure and assesses what happens to their health over time. It is a desirable design because exposure precedes the health outcome — a condition necessary for causation — and is less subject to bias because exposure is evaluated before the health status is known. The cohort study is also expensive, time-consuming and the most logistically difficult of all the studies. It is most useful for relatively common diseases. To assess suitability, we find out the commonality of the disease we wish to study. Does it occur in 10 percent of all births, 1 percent of births, or 0.001 percent of births? For example, if low weight occurs in 10 percent or more of all births, then we might investigate a relatively small group of newborns, say 200 to 400, and characterize…...

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