So if looking at contemporaneous economic conditions is not a reliable way to judge presidents, how should they be graded?

Consider how you would assess a physician whom you could observe treating only a single patient. A terrible way to judge the doctor is by whether the patient lives or dies. After all, even the best physicians have patients who die when their illnesses are severe. And given the natural restorative powers of the human body, even a quack can have a patient who returns to good health.

The best way to evaluate a physician is to determine whether state-of-the-art medical practices are followed. A doctor who prescribes the right antibiotics gets high marks, while one who prescribes snake oil gets a failing grade.

Similarly, a better way to judge presidents is by the policies they pursue, not the outcomes over which they preside. This task is harder than merely looking at unemployment, inflation and the growth of gross domestic product. It requires having a view about what policies are best at fostering prosperity and acknowledging that the experts are often divided on that question.

Looking ahead, judging presidents by policies rather than outcomes may be all the more important. In a new book, “The Rise and Fall of American Growth,” the economist Robert Gordon argues that we are in the midst of an era of meager technological change. Yes, we now have smartphones and Twitter, but previous generations introduced electric lighting, indoor plumbing and the internal combustion engine. In Mr. Gordon’s view, technological change is just not what it used to be, and we had better get used to slower growth in productivity and incomes.

I have no idea whether Mr. Gordon’s pessimism is justified. In light of economists’ abysmal track record of long-term forecasting, all such prognostications should be taken with a shaker of salt.

But if Mr. Gordon turns out to be right, future presidents will operate in a more difficult economic environment than many of their predecessors. Because the pace of scientific and technological advance has a life of its own, slow productivity growth may be a problem without a solution.

Particularly in this environment, judging presidents based on outcomes rather than policies is an egregious error. We would end up blaming them for results over which they have little control. We would find ourselves disappointed in our leaders, even if they are doing the best they can. Instead of supporting sound policies, we might cast our votes out of a frustration that is not fully rational.