Experimental design is a major challenge in science. Attempting to strike the right balance between testing the thing that you want to test, and testing the thing that you can test, is very tricky. This is further complicated by the fact that the controlled laboratory environment might be necessary for your experiment to work reliably, but that might undermine its relation to the real-world phenomenon you’re attempting to learn about. These ideas are captured with the concepts of internal and external validity, as explored in this video. This video is aligned with Chapter 3 of the book Recipes for Science: An Introduction to Scientific Methods and Reasoning, by Angela Potochnik, Matteo Colombo, and Cory Wright.