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 them with respect to their exposures during pregnancy. We would expect to see 20 to 40 low-weight babies in this group. We would want to know if, while pregnant, their mother's exposure to electric and magnetic fields was different from the other 180 to 360 births.
The cohort study approach is good for our hypothetical study because we can identify a number of pregnant women, characterize their exposure during almost their entire pregnancies and assess the babies' weights at birth. Thus we limit the possibility of investigator preferences, or "bias," affecting the selection of study subjects. We must make sure nearly everyone selected for our study participates, because those not participating may be different from those who do, causing another type of bias. The assumption of risk (such as exposure to electric and magnetic fields during pregnancy) necessarily precedes the outcome (birth), and that is a necessary condition for inferring a cause.
Finally, because we selected our study subjects on the basis of their exposure only, the cohort approach enables us to look at other pregnancy outcomes such as birth defects, spontaneous abortion or increased mortality, in addition to birth weight.

Case-Control Study
The "why me?" study investigates the prior exposure of individuals with a particular health condition and those without it to infer why certain subjects, the "cases," become ill and others, the "controls," do not. The main advantage of the case-control study is that it enables us to study rare health outcomes without having to follow thousands of people, and is therefore generally quicker, cheaper and easier to conduct than the cohort study.
One primary disadvantage of a case-control study is a greater potential for bias. Since the health status is known before the exposure is determined, the study doesn't allow for broader-based health assessments, because only one type of disease has been selected for study. If the condition we wish to study is rare — for instance, affecting less than 5 percent of the population — the cohort approach would not identify enough subjects from which to draw statistically reliable inferences, unless we looked at a very large number of subjects. For example, in our study of low birthweight, if between 500 and several thousand births would be needed to get 10 to 50 low-weight births, developing exposure information for all of those births would be cumbersome, expensive and time-consuming.
In the case-control design, we can review birth certificates of several thousand newborns and find a certain number that exhibited low birthweight — 50 to 100, for example — and a comparable number of normal birthweights, and compare them with respect to electric and magnetic field exposures. This approach has the advantage of identifying a sufficient number of cases of the rare outcome we wish to study out of a population of thousands of births. It then requires us to develop exposure and risk factor data only for the limited number of individuals in our study. Thus, it is quicker, easier and less expensive than the cohort design, which would require such information for all of the several thousand births. It is an approach commonly used for studies of cancers and other rare diseases. Also, because subjects were selected on the basis of outcome only, we can evaluate a variety of exposures, such as electric fields, magnetic fields, chemical exposures and so forth.
The case-control study has the disadvantage of selecting cases and controls after both the outcome and the assumption of risk have occurred. This makes substantial bias a possibility because we may inadvertently favor certain births for inclusion in our study, and because certain women who should have been eligible for our study were not (those pregnant mothers whose fetus spontaneously aborted, for instance). Once chosen on the basis of one outcome (low birthweight), our subjects cannot be analyzed for certain other outcomes (spontaneous abortion), as they could in a cohort study.
Another consideration in choosing an epidemiological design is the commonness of the risk factor. Common exposures can be studied by either the cohort or case-control design. Rare exposures are best studied by the cohort method since groups are selected on the basis of their exposure status.
Consider our study at hand. Most of us are exposed at home to very low magnetic fields — under 10 milliGauss (mG). But some homes score as high as 40 mG, or higher, and some occupations measure exposures in the hundreds or thousands of mG. Let's define exposed houses as those having at least 10 mG. If, in

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