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Epidemiology is the study of how disease is distributed in populations and the factors that influence or determine this distribution. Clinical epidemiology denotes the application of epidemiologic methods to questions relevant to patient care and provides a highly useful set of principles and methods for the design and conduct of quantitative clinical research. Validly analyzing, correctly reporting, and successfully interpreting the findings of a clinical research study often require an understanding of the epidemiologic terms and measures that describe the patterns of association between the exposure of interest (treatment or intervention) and a health outcome (disease). This statistical tutorial thus discusses selected fundamental epidemiologic concepts and terminology that are applicable to clinical research. Incidence is the occurrence of a health outcome during a specific time period. Prevalence is the existence of a health outcome during a specific time period. The relative risk can be defined as the probability of the outcome of interest (eg, developing the disease) among exposed individuals compared to the probability of the same event in nonexposed individuals. The odds ratio is a measure of risk that compares the frequency of exposure to a putative causal factor in the individuals with the health outcome (cases) versus those individuals without the health outcome (controls). Factors that are associated with both the exposure and the outcome of interest need to be considered to avoid bias in your estimate of risk. Because it takes into consideration the contribution of extraneous variables (confounders), the adjusted odds ratio provides a more valid estimation of the association between the exposure and the health outcome and thus is the preferably reported measure. The odds ratio closely approximates the risk ratio in a cohort study or a randomized controlled trial when the outcome of interest does not occur frequently (<10%). The editors, reviewers, authors, and readers of journal articles should be aware of and make the key distinction between the absolute risk reduction and the relative risk reduction. In assessing the findings of a clinical study, the investigators, reviewers, and readers must determine if the findings are not only statistically significant, but also clinically meaningful. Furthermore, in deciding on the merits of a new medication or other therapeutic intervention, the clinician must balance the benefits versus the adverse effects in individual patients. The number needed to treat and the number needed to harm can provide this needed additional insight and perspective.