A school district wanted to understand how family income affects student performance on standardized tests like the SAT. So they collected information from a random sample of test takers over a previous five year period, and examined the relationship between family income and SAT score over those prior years.

1. If there is a relationship between family income and SAT score, why can't the school district conclude that differences in score are caused by differences in parental income (Hint: Think about the difference between an observational study and an experiment)?