Epidemiology is the study of the distribution and determinants of health-related states or events in populations and the application of this study to control health problems. The basic measurements used in epidemiology include rates, ratios, and proportions to describe the occurrence of mortality, morbidity, disability, and other disease attributes in populations. Rates express the frequency of events over time, proportions express the relationship between parts and the whole, and ratios compare two rates or quantities. These measurements are essential tools for epidemiologists to investigate disease causation, describe population health status, and evaluate interventions.