Jim Shelton, who heads CZI’s education work, said in an interview that the organization relies on a great deal of other research, but highlights Bloom to illustrate in the best case scenario for what schools might accomplish.

“It stands to reason that many kids that currently perform at levels that we consider average or even below average could be performing at levels that we would consider superlative,” he said.

The conclusions on the effects of tutoring from Bloom’s widely cited paper are drawn from two studies conducted by graduate students at the University of Chicago.

In both studies, students were taught novel subject matter—probability or cartography—using different methods over the course of a few weeks. Some students were taught in a traditional lecture style, others received “mastery-based” teaching, and others received small group tutoring.

On a final test, students who were tutored one-on-one or in small groups came out far ahead, and in some cases the average tutored student beat 98 percent of those taught in the traditional way. Students who received the mastery-based teaching—which overlaps with modern conceptions of personalized learning—also did much better, though not as well as those tutored.

The applicability of these studies today is an open question. Combined, the studies focus on just three schools and a few hundred students. And since this was done more than 30 years ago, things like what traditional instruction looks like may have substantially changed.

The papers include little information about those final tests, but it appears they were designed by the researchers, unlike a traditional standardized test. Researcher-created assessments on subjects that are totally new to students—like cartography and probability, in this case—tend to see students make the largest gains.

Bloom’s work also doesn’t focus on technology-based tutoring, a point personalized-learning advocates usually acknowledge. “If it supports anything, it supports one-on-one human tutoring,” Riley said.

But what earned the most attention, then and now, is how big of an impact tutoring had on students. The difference between tutoring and traditional instruction after just three weeks was two standard deviations—to researchers, a truly incredible result. It means bringing students from average to exceptional.

“I’ve never seen a study in education that found effects in the range of two standard deviations, so it’s remarkable for that reason,” said Jon Guryan, a Northwestern professor who has done research on tutoring.

Another researcher, Robert Slavin of Johns Hopkins University, logged concerns about Bloom’s outsize claims as early as 1987. Focusing on such unusually large gains, he wrote, “is misleading out of context and potentially damaging to educational research,” since it could lead researchers to “belittle” more realistic results.