A professor wanted to know whether a prep course would increase studentsâ€™ test scores.

Twelve subjects were randomly chosen to take the prep course, and they scored an average of $20.3$ with a standard deviation of $8.3\text{ points}$. Another $12\text{ subjects}$ were randomly chosen who were not allowed to take the test prep course, and they scored an average of $18.3$ with a standard deviation of $9.1\text{ points}$.

Despite these scores, why might the professor not be convinced that the course helps?