In addition to surveys that are cross-sectional, as seen in many examples in earlier chapters, study data may be collected in many different ways. For example, investigators are faced more and more frequently with the problem of determining whether a specific factor or exposure is related to a certain aspect of health. Does air polution cause lung cancer? Do birth control pills cause thromboembolic death? There are reasons for believing that the answer to each of these and other questions is yes, but all are controversial; otherwise, no studies are needed. Generally, biomedical research data may come from different sources, the two fundamental designs being retrospective and prospective. But strategies can be divided further into four different types:
1. Retrospective studies (of past events)
2. Prospective studies (of past events)
3. Cohort studies (of ongoing or future events)
Retrospective studies of past events gather past data from selected cases, persons are identified who have experienced the event in question, and controls, persons who have not experienced the event in question, to determine differences, if any, in exposure to a suspected risk factor under investigation. They are commonly referred to as case-control studies; each case-control study is focused on a particular disease. In a typical case-control study, cases of a specific disease are ascertained as they arise from population-based registers or lists of hospital admissions, and controls are sampled either as disease-free persons from the population at risk or as hospitalized patients having a diagnosis other than the one under study. An example is the study of thromboem-bolic death and birth control drugs. Thromboembolic deaths were identified from death certificates, and exposure to the pill was traced by interview with each woman's physician and a check of her various medical records. Control women were women in the same age range under the care of the same physicians.
Prospective studies of past events are less popular because they depend on the existence of records of high quality. In these, samples of exposed subjects and unexposed subjects are identified in the records. Then the records of the persons selected are traced to determine if they have ever experienced the event to the present time. Events in question are past events, but the method is called prospective because it proceeds from exposure forward to the event.
Cohort studies are epidemiological designs in which one enrolls a group of persons and follows them over certain periods of time; examples include occu pational mortality studies and clinical trials. The cohort study design focuses on a particular exposure rather than a particular disease as in case-control studies. There have been several major cohort studies with significant contribution to our understanding of important public health issues, but this form of study design is not very popular because cohort studies are time and cost consuming.
In this chapter we focus on study designs. However, since in biomedical research, the sample survey is not a common form of study, and prospective studies of past events and cohort studies are not often conducted, we put more emphasis on the designs of clinical trials which are important because they are experiments on human beings, and of case-control studies, which are the most popular of all study designs.
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