Hundreds of statistical tests, procedures, and descriptive measures are used in clinical research. Risks, odds, and hazards are among the most common but not always the most understood. They are often used in all three types of observational studies in medicine and epidemiology – case-control, crosssectional, and cohort studies – so a goodunderstanding of what they are and are not is helpful in understanding these studies. Here, I briefly describe these measures, how they are used in observational studies, and how to interpret them.