Epidemiology

Epidemiology is the study of disease patterns in the population and the risk factors that influence those patterns. Epidemiologists are concerned with the who, when, and where of disease occurrences. That is, epidemiologists investigate (1) whether persons who have a disease are different from persons who do not get the disease based on traits or characteristics of interest, (2) whether the occurrence of disease has increased or decreased over time, and (3) whether disease occurs more frequently in one geographic area than another.

Two hallmarks of epidemiologic studies are the focus on groups of people rather than individuals and the inclusion of a comparison group. Epidemiologic studies are either observational or experimental. In observational studies the epidemiologist does not have control over assignment to study groups. Retrospective, cross-sectional, and prospective studies are the three types of observational studies. Retrospective and cross-sectional studies are similar in that group membership is determined by the presence or absence of the disease in question and then information is obtained about characteristic(s) and/or exposure(s) of interest among group members. These types of studies differ in that retrospective studies obtain data on past exposures, whereas cross-sectional studies obtain information on current exposures or characteristics. Prospective studies differ from both retrospective and cross-sectional studies in that groups are identified based on the presence or absence of the characteristic(s) or exposure(s) of interest and then are followed over time to determine who gets disease.

One example of a classic, prospective epidemiologic study is the Framingham Heart Study, a study of risk factors for cardiovascular disease (CVD) that was initiated in 1948 among residents of Framingham, Massachusetts. Residents between ages 30 and 62 years were eligible to participate and 6,500 persons were randomly selected from the community and invited to participate in the study. Assessments of factors thought to be related to heart disease (e.g., blood pressure, weight, cholesterol, smoking) were made every 2 years over a 20-year interval and new cases of and/or deaths from CVD were recorded. Investigators then examined the associations between the characteristics of interest and the risk of developing CVD among men and women during the follow-up period. The Framingham Heart Study is highly regarded for the important information about CVD risk factors that it provided. For example, it was this study that established that risk of CVD increases with increasing levels of total serum cholesterol.

In experimental studies the epidemiologist controls the assignment of study participants to the “exposed” or “unexposed” group. Community trials and clinical trials are the two types of experimental studies. In community trials (also called field experiments) the group as a whole is studied collectively and group assignment is not randomized (nor is randomization possible). A classic example of a community trial is the addition of fluoride to the water supply of a city to examine whether fluoride reduces the likelihood of getting dental caries. In this case, the comparison group is a community whose water supply is naturally low in fluoride and is not supplemented. In clinical trials, individuals rather than communities are studied and participants are randomized either to receive the treatment of interest or to a control condition in which no active treatment is given. Experimental and control groups in clinical trials must be as similar as possible except with respect to the treatment of interest. This is accomplished through the randomization process, which makes it possible for the investigator to avoid introducing biases into the study and ensures that participants will be comparable on both known and unknown factors that could influence the outcomes of interest. Clinical trials can be therapeutic trials, which test the effectiveness of specific therapies in persons with disease; intervention trials, in which an intervention is given to persons who have not yet gotten the disease but are at increased risk of the disease because of certain characteristics they have; or prevention trials, which test the effectiveness of preventive therapies or procedures. The primary difference between clinical trials and prospective studies is that clinical trial participants are randomly assigned to the experimental or control group. This also is the greatest strength of clinical trials, which are often considered the “gold standard” by which to judge the effectiveness of treatments for numerous diseases.