Epidemiology and epidemiologic methods: an introduction for respiratory care practitioners.

Epidemiology is the study of the distribution of disease in a population and of the factors that determine that distribution. Respiratory care practitioners encounter many aspects of epidemiology in training and practice and should become aware that most initial epidemiologic observations are made by clinicians. The determinations that excessive oxygen administration caused retrolental fibroplasia and that exposure to cotton bracts in the textile industry caused byssinosis resulted from the collaborative efforts of epidemiologists, laboratory workers, and clinicians. The three types of study designs in observational epidemiology are: (1) the cohort (incidence) study, in which a study factor is observed at the onset of the study and the study population is followed over time for the development of new cases of a disease; (2) the cross-sectional (prevalence) study, in which the presence or absence of risk factors is measured at one point in time in a study population and prevalence rates of disease among those with and without the risk factors are compared; and (3) the case-control study, in which the numbers of persons with and without a given disease who share certain background characteristics or exposure levels to risk factors are determined and compared. The epidemiologist's continued collaboration with the clinician and the laboratory scientist is important because epidemiology alone can rarely provide conclusive solutions to every aspect of a problem.