This short lecture from Yale's online course Understanding Medical Research: Your Facebook Friend is Wrong explains the cohort study, one of the basic designs used in epidemiology. The instructor walks through how researchers select a group of people, split them by exposure to a suspected risk factor, and follow them forward in time to see who develops a given outcome, tracing the logic from tobacco and lung cancer studies as a model case. The lecture covers the difference between prospective and retrospective cohorts, how researchers calculate relative risk, and where confounding variables can creep in and distort the results. It is aimed at a general audience trying to read health news and medical studies more critically rather than at specialists, and it stays close to definitions and worked reasoning rather than technical statistics.