Types of clinical studies. II. Cohort studies.

Cohort studies include groups of patients that are followed over time to determine the incidence or natural history of a disease, together with the risk factors for specific outcomes. These studies can be used to determine disease characteristics as well as the causal relationship between exposure and disease. They are, however, very expensive, because they need large patient samples. Cohort studies can be classified as prospective (the investigator defines the sample and measures predictive variables before outcomes arise) or retrospective (the investigator defines the sample and measures predictive variables after outcomes arise). Cohort studies are susceptible to various types of bias (systematic errors) that must be eliminated in order to ensure the validity of the results. To increase the validity of cohort studies the investigator has to consider exposure, outcome, sample selection and the statistical analysis (risk ratios and/or differences, absolute and relative risks, etc.).