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. Record abstracts are used to extract information from written records kept for another purpose.
Physical examinations require more exact recording of findings than in a strict clinical setting.
Subjects with no abnormal findings must be fully examined and described to ensure comparability within the study. Clinicians must be trained and adequate quality control utilized to ensure minimal variation. Laboratory components of epidemiologic studies require collecting biologic specimens from study participants. Every word in a question can influence the validity and reliability of the responses. Collecting data by questionnaires can either be by interview or by self-administration.