There is an inherent danger in any factor that can be calculated to an impressive number of digits. I have worried about instances in which a teaching score of 4.21 is thought truly superior to a 4.10. We sometimes look for hard numbers to justify soft judgments. The good faculty, and this is the overwhelming majority, do not. The U. Wash. study reports that the simple factor of showing greater enthusiasm in class brings up the ratings of many categories, and even makes the text book seem better.
Conventional wisdom is that easier graders earn better student scores.
This theory has the advantage of being almost impossible to prove right or wrong, due to the associated factors that complicate simple comparisons. Let's take numbers here at CLAS as an example, based on a recent semester's course grades and student evaluations. But before too much is made of these data, the differences in scores are small and not compelling as to conclusions. Furthermore, the correlation between departmental GPAs and the evaluation scores is far from consistent. In principle, peer review should be the most reliable assessment of faculty teaching - professionals judging professionals.