Augusto wants to prove to his parents that watching tv while he does his homework will him get better grades.
he decides to ask 20 of his friends if they watch tv while they do their homework and what their grades are.
he finds that his friends who have high grades also watch tv while they do their homework.
he tells his parents that he has proven that watching tv while doing homework will cause his grades to improve.
what mistake has augusto made?

This guy has 2 major problems.

First, for sample investigation, the number of samples is 20, which is too small to draw the conclusion.

Second, his evidence does not prove the relationship between high grades and watching TV. Because he does NOT exclude other factors, such as hard-working, asking questions, etc. He needs to prove it by: let the guy watch TV at the beginning of one course, and does not spare any other effort on to it, only TV. If he gets decent grades, then it is proved