Regression toward the mean

Regression toward the mean is when something that happened before that was either really big (like a high number) or really small (like a low number) is less likely to happen in the same way again. For example, if a student got a really bad grade on a test, it's very unlikely that they will get another really bad grade on their next test. In other words, their grades are likely to move back closer to the average—or mean— grade for the class.