Although observational studies are frequently used for descriptive purposes, they also are used in discov¬ering the etiologic factors of disease. By observing disease rates in groups of people who are differentiated on the basis of an experience or exposure, the investi¬gator can begin to understand the factors that contrib¬ute to disease. To illustrate, differences in the rates of i disease may occur in the obese compared with the nonobese, in smokers compared with nonsmokers, and I in those with high levels of life stress compared with t those with low levels. Variables that define characteristics. obesity, smoking, and stress are called exposure
variables.