Epidemiological study design and the advancement of equine health.

The primary purposes of epidemiological investigations are to learn about causal mechanisms related to disease incidence and identify factors for therapy and prevention. Epidemiological studies can be observational--further categorised as descriptive or analytical--or experimental. Investigators performing experimental studies, or randomised controlled trials (RCTs), randomly assign treatments or exposures to study participants for the expressed purpose of the study. The most frequently encountered observational epidemiological studies employed to investigate issues of equine health are cohort, case-control, cross-sectional and case series. A cohort study is an investigation in which the researcher follows (observes) a group, termed the cohort, over time to measure incidence of a particular outcome. A case-control study is an investigation in which the researcher selects a group of affected individuals (cases) and a comparison group (controls) to investigate factors associated with being a case. A cross-sectional study investigates a group of individuals for study that is often defined by membership in a target population and data concerning outcome and exposure are either collected related to the same time-point or data are collected by investigators at the same time. Case series are descriptive studies used to generate hypotheses concerning predictors of disease or recovery that can be performed retrospectively or prospectively. The best evidence for clinical practice is often derived from patient-centred observational epidemiological studies and it is imperative that equine veterinarians become familiar with study designs for the appropriate interpretation of epidemiological findings.