When only the wealthy could afford education, tests and exams did not evaluate the students. Instead, tests judged teachers for parents who wanted to know whether their children learned enough to justify the instructors' wages. Obviously, this is different today. Now, such evaluations are used to distinguish between high-achieving and low-achieving students, filtering them into careers and other opportunities.

Given the significance of this process, a great deal of effort is invested in developing accurate student assessments. Fair assessments have a large number of questions that cover a wide breadth of topics and reward true knowledge over lucky guesses. A good assessment also requires students to think about problems they have never faced before. Correct responses therefore reflect an ability to generalize knowledge more broadly.

The process of evaluating machine learning algorithms is very similar to the process of evaluating students. Since algorithms...