Alma wants to prove to her parents that listening to music while she studies for her tests will help her get better grades. She decides to ask 15 of her friends if they listen to music while they study for tests and what their grades are. She finds that her friends who have high grades also listen to music while they study for tests. She tells her parents that she has proven that listening to music while studying for tests will cause her grades to improve. What mistake has Alma made?