EXAMPLE 12 Evaluation Correlation?

Grades that students earn in courses are correlated positively with the ratings that students give on anonymous end-of-course surveys administered by the university. One very simple interpretation is that instructors give easy tests with “low standards,” which in turn causes students to express appreciation through high instructor ratings. But perhaps there is a third variable that drives the other two variables: A professor who is a skillful teacher and motivator may be more likely both to be rated well and to inspire high performance. Or perhaps courses that have higher grade distributions are more likely to be upper-level courses for majors in that subject, and such students would be more prepared for and favorably inclined toward the course.