Imagine you are taking an Algebra test. You’re about halfway through when you come across a diagram of a human cell. The question is asking you to label the different parts of the cell. You’d probably realize pretty quickly that this question isn’t a valid test of your Algebra knowledge. It’s just not testing what it claims to be testing.
Unfortunately, this type of thing happens all the time in studies. Researchers are constantly in danger of using metrics that aren’t actually measuring what they think they are measuring. And that’s why knowing how to test a study for validity is so crucial. It helps you determine which results to trust and which results to suspect.