In one of many testy exchanges that took place at the Michigan billionaire’s hearing Tuesday evening, Democratic Senator Al Franken, of Minnesota, started off with a wonky-sounding prompt that could’ve easily caused many viewers to tune out: “I would like your views on the relative advantage of assessments and using them to measure proficiency and growth,” he said, noting that he’s a staunch advocate of focusing on growth.

DeVos’s response ended up being one of the highlights of the night: “I think, if I’m understanding your question correctly around proficiency, I would correlate it to competency and mastery, so each student is measured according to the advancements they are making in each subject area.” But what she defined was growth—not proficiency. “It surprises me that you don’t know that issue,” Franken said.

In response to a question about DeVos’s understanding of the growth-proficiency debate, a transition spokeswoman pivoted to the role of data and technology in measuring student learning and said that the nominee had been “cut off.” “Mrs. DeVos was suggesting that, in fact, there are many more strategies being deployed today in classrooms by savvy educators that inform where students are and where they need to be,” the spokeswoman wrote in an email Thursday.

The exchange between DeVos and Franken has caused a lot of brouhaha on social media and has been covered extensively by the news media. “DeVos had no idea what Franken was talking about,” wrote Vox’s Libby Nelson. “This wasn’t a matter of mixing up some jargon. DeVos’s response, as well as her reactions to similar questions about the basics of federal education policy, suggested she knows little about what the department she hopes to lead actually does,” she wrote. The Slate contributor Dana Goldstein pointed out that DeVos’s inability to set the record straight is one of many reasons Democrats begged, unsuccessfully, for more than five minutes of questioning time per senator.

The 74’s Matt Barnum took the opportunity to explain why the growth-versus-proficiency debate is so critical when assessing how to best hold schools accountable and, ultimately, improve the learning outcomes of all students. He cited a 2008 survey showing that more than two in three education researchers believe growth is a good way to measure school quality. (Just 9 percent said that “raw test scores”—i.e., proficiency alone—is the preferred tactic.)

Why are researchers, at least in this survey, so in favor of growth measures? Perhaps the most basic reason is that there are many factors that affect what level a student achieves at and whether they hit the bar set at proficiency. Careful research finds that about 20 percent, and perhaps less, of the variation in student achievement is explained by differences in schools. That pales in comparison to out-of-school factors, like poverty, that have a significant effect on learning. Schools matter, but they aren’t the sole or even main driver of student outcomes. What that means for proficiency is that schools that take disadvantaged students—those in poverty, those who come in at low achievement levels—will look much worse. The school could be doing a great job helping kids improve, but if they start out at a very low level, that might not show up on proficiency measures.

I decided to get a better sense what states are doing today—which ones are still focusing mainly on proficiency and which ones have embraced growth—to lend some context to the debate and illustrate why knowing DeVos’s position is so critical.