A quarter of a century ago, barely half the children of primary school age in sub-Saharan Africa were enrolled in school. By 2012 the share was 78 percent. In South Asia, primary school enrollment jumped to 94 percent from 75 percent over the same period.

This didn’t happen by chance. Policy makers around the world have come to understand the importance of learning for every aspect of human development. Universal primary education was one of the United Nations’ core Millennium Development Goals, which mobilized large amounts of aid in the first decade of the century for poor countries to expand access.

Despite this phenomenal advance, however, a peek under the headline statistics suggests that much of the world has, in fact, progressed little. If the challenge was to provide a minimum standard of education for all, what looks like an enormous improvement too often amounted to a stunning failure.

“We’ve made substantial progress around the globe in sending people to school,” said Eric Hanushek, an expert on the economics of education at Stanford University. “But a large number of people who have gone to school haven’t learned anything.”