The research shows that although tougher professors don't give out as many A's and B's, and their student evaluations are not as good, their students do better when they get to upper level (junior and senior level) classes.
It can be hard to find this out. A study like this needs "ceteris paribus" conditions. That is, all other factors have to be held constant (something discussed in the first chapter of probably all principles of economics texts). For example, if good students intentionally take tough profs, then we can't be sure why they did better later. Was it because they were better students or because the profs were tough and upheld high academic standards?
But at the Air Force Academy, here is why they have "ceteris paribus" conditions:
"All students at the academy are required to take a common core of 30 credits. No matter how much they might hate Calculus I, they still have to take Calculus II. Most course sections are small—about 20 students—and students have no discretion in choosing their sections or instructors. Finally, every Calculus I section uses the same common tests, which are graded by a pool of instructors. (One instructor grades Question 1 for every section, another instructor grades Question 2, and so on.)
All those factors make the Air Force Academy a beautifully sterile environment for studying course sequences.
Mr. West and Mr. Carrell (the economists who did the study) didn't have to worry that their data would be contaminated by students self-selecting into sections taught by supposedly easy instructors, or male instructors, or any other bias. They didn't have to worry about how to account for students who never took the follow-up courses, because every student takes the same core sequence. And they didn't have to worry about some instructors subtly grading the tests more leniently than others."