There are many ways to judge a president—polls, approval ratings, legislative successes, foreign breakthroughs, memorable speeches, and historic moments. But there’s a better way than any of these, and Fred Greenstein, a professor of politics emeritus at Princeton University, has developed it.

Rather than rely on what others think, the Greenstein method is to judge a president by his performance in office—that is, his effectiveness. Put another way, does the president offer strong and persuasive leadership? Polls and ratings don’t always capture this.