Imagine you are at your doctor, and she wants to test you for a rare medical condition. This is a little scary, and even scarier when the test comes back positive. But, before you panic, it’s worth asking: What is the probability that you actually have the condition? This question can be answered using Bayes’ Theorem. In this video, we walk through how. This illustrates how medical tests and other diagnostics only give probabilistic information; they have false negatives and false positives. This also illustrates the importance of base rates for probabilities. Finally, this kind of question is a really good application of Bayesian statistics. This video is aligned with Chapter 8 of the book Recipes for Science: An Introduction to Scientific Methods and Reasoning, by Angela Potochnik, Matteo Colombo, and Cory Wright.