Part of Yale's open course Understanding Medical Research: Your Facebook Friend is Wrong, this short lecture explains how diagnostic tests are evaluated statistically. The instructor walks through sensitivity, specificity, and predictive value, showing how a test's accuracy changes depending on how common a disease is in the population being screened. Using simple numeric examples, the lecture demonstrates why a positive result on a highly accurate test can still be more likely wrong than right when the underlying disease is rare, a point aimed at helping viewers read health headlines and screening claims more critically. The pace is brisk and example driven, built around a single teaching goal rather than a survey of the field. It functions as one module in a broader course on interpreting medical research claims, but stands on its own as a clear primer on the math behind diagnostic testing.