Question

• What kinds of epidemiologic research studies would you conduct to further explore the issue of cigarette smoking among middle school and high school students? Identify several study designs.

Answer 1

Four Most Common Types of Epidemiological Studies
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.

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. The case-control study has the disadvantage of selecting cases and controls after both the outcome and the assumption of risk have occurred. 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.

Occupational Epidemiological Study
The occupational study can be designed using any standard epidemiologic design, simply selecting working people with particular jobs or exposures as subjects. The main advantage of this approach is that workers often have substantially higher exposures to certain risk factors than the typical population, which increases our chances of detecting an effect if one truly exists. The main disadvantages are that workers with various jobs differ substantially from one another in terms of risks, and that the working population is substantially different from the nonworking one (such the rich, elderly or disabled), making it difficult to generalize to populations with some nonworking people. We usually look to occupational settings to exploit situations of high exposure. The number of eligible subjects in these settings is smaller than in the general population, but that is more than balanced by the extreme levels of exposures often seen in the workplace, which increase our chances of seeing effects. There are two caveats to occupational epidemiological studies. First, in the workplace, people are exposed to a variety of risk factors that may affect results. For example, many workers are exposed to a variety of chemicals (such as solvents) that are known or suspected carcinogens; to a variety of electric and magnetic fields at different intensities and frequencies; and to other factors such as stress and poor ventilation or air quality. Second, the number of people exposed to the risk factor we're interested in may be much smaller in the workplace than in residences, so in some cases it may be difficult to identify a sufficient number of exposed workers. And it may be difficult to identify a comparison population of workers not exposed to the risk factor (or exposed at a substantially lower level) who also have comparable characteristics with respect to other possible risks.

Cross-Sectional Study
The "Am I like my neighbors?" study compares groups in terms of their current health and exposure status and assesses their similarities. The main advantage is that the cross-sectional study is a particularly easy study to conduct, as we do not have to wait for the health outcome to occur or estimate what the level of exposure was likely to have been years ago. Its main disadvantage is that a cause can't be inferred, because only current health and exposure are being studied. The cross-sectional study is the one in which we assess a group's health status and exposure status simultaneously. We might inquire about recent health problems (including breast cancer diagnosed in the past year) and assess the current electric and magnetic fields exposures in people's homes as part of the same survey. An important limitation of this approach is that it does not allow for changes over time, and thus cannot accommodate diseases that take time to develop.