Epidemiology is the study of the health status of populations.
The populations studied can be human or animal and the health status can include disease or determinants. Regardless of the subject, data collection methods vary. Having identified the researchable problem, the population and the variables to be measured, one must choose the procedure for collecting data and the measuring instrument to be used. Instruments are simply devices for measuring the variables of interest. Questionnaires are the mainstay for epidemiologists though the other methods can be just as useful and each has its strengths and limitations.