The proportion of revenue a charity spends on programs, as opposed to fundraising and administration, has become a key measure of effectiveness, but how a group spends that mission money is far more important, a New York Times columnist argues.

“Overhead does matter. But it is dwarfed by a different question: Is this group’s work effective?” writes Tina Rosenberg, a Pulitzer Prize-winning author, in the Times’s Opinionator blog.

Ms. Rosenberg cites analyses by Oxford University philosophy researcher Toby Ord and New York watchdog group GiveWell of how charities can achieve the greatest impact. She characterizes as most effective organizations that tackle the most serious problems and employ strategies that reach the most people—noting, for example, that the estimated $42,000 cost of training a guide dog for one blind person could provide eye surgeries for some 1,300 sight-impaired people in Africa.