Senate Democrats hit Betsy DeVos, Donald Trump’s pick for education secretary, on all sides at her confirmation hearing Tuesday night. But the question that made her look worst required her to demonstrate a basic understanding of education policy — a test DeVos failed.

Sen. Al Franken asked DeVos to explain her thinking on whether test scores should be used to measure students’ proficiency or their growth. That’s an important, and basic, difference because it affects how schools are labeled as succeeding or failing.

But DeVos had no idea what Franken was talking about.

“I think if I am understanding your question correctly around proficiency, I would correlate it to competency and mastery, so each student according to the advancements they are making in each subject area,” she said to Franken.

“That’s growth,” Franken retorted, correctly. “That’s not proficiency.” By the time DeVos understood Franken’s question, she had no time left to answer.

This wasn’t just 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.

Why the proficiency-versus-growth debate matters

Franken’s question wasn’t as aggressive as other Democrats’ lines of inquiry about the DeVos family political donations or Trump’s conflicts of interest. But it was relevant, because DeVos’s opinions on proficiency and growth are about to matter for every public school in the United States.

The proficiency-versus-growth debate is about how schools should be held accountable for their students’ test scores. “Proficiency” means judging whether kids hit a benchmark, such as whether they can read at grade level. “Growth” means judging how much progress they’ve made — if they started the school year behind, are they catching up even if they’re not at grade level yet? If they were already proficient, did they make progress and learn more, or just tread water?

A school that looks fine if you’re only measuring proficiency might not look as good if you measure growth. And a school that isn’t hitting proficiency benchmarks could still be helping students make fast progress that shows up in a growth measurement.

No Child Left Behind, the federal education law in effect from 2002 to 2016, required schools to measure proficiency (but not growth) and penalized them if their students weren’t up to par. The Every Student Succeeds Act, the law Congress passed in late 2015 to replace No Child Left Behind, gives states more freedom decide how to hold schools accountable.

The idea that growth and proficiency are both important, but that growth was key to capturing what proficiency misses, came up frequently in the recent debate over rewriting the law.

Here’s where DeVos comes in: The next education secretary will have to decide whether to rewrite the Obama administration’s rules for what those state requirements for schools should look like, and will also have to sign off on states’ plans to hold schools accountable. Right now, those plans have to include a measurement of proficiency and can also include a measurement of growth.

Franken’s question suggested a strong preference for growth as a better measurement — a position he shares with Arne Duncan, President Obama’s first secretary of education, but also with the conservative Fordham Institute. Still, he wasn’t asking if DeVos agreed with him. He was just asking if she understood the difference.

DeVos flubbed several questions about what the Education Department actually does

DeVos is well-known in education circles as a supporter of school vouchers, which allow students to use public money to attend private schools, and, to a lesser extent, for her role in Michigan’s loosely regulated charter school sector.

That experience made DeVos a natural choice for Republicans, who argue that allowing poor students to attend private and religious schools is both good policy and a matter of civil rights. They argue that rich parents can effectively choose their children’s school by buying a house in a good school district, and poor parents deserve the same choices.

But while DeVos’s voucher support fits comfortably in the Republican mainstream, it might not be all that relevant to her time in office should she be confirmed, unless Congress is inclined to pass Trump’s multibillion-dollar voucher plan.

On the other hand, DeVos is guaranteed to be handling many issues she has less experience in. The Education Department hands out billions of dollars of grants and loans to college students. It gives grants to help educate students with disabilities and poor students. It investigates complaints that schools violated students’ civil rights, which under President Obama has included aggressive investigation of colleges accused of mishandling sexual assault.

And when DeVos was asked about those issues, she often floundered. She didn’t seem to understand how the Individuals With Disabilities Education Act, the bedrock federal law guaranteeing an education to students with disabilities, works — and later she admitted to Sen. Maggie Hassan that she might have been confused. She was way off on a figure about how much student debt has grown. She gave a non-answer to Sen. Bob Casey’s question about the Education Department’s instructions to colleges on handling sexual assault, suggesting she knew the guidance was controversial but didn’t understand what it was.

Few education secretaries are conversant with everything the department does on their first day, or even much later. In his seven years as secretary, Duncan stuck firmly to talking points on higher education policy while speaking much more fluently on K-12, where he had much more experience.

Even by those standards, though, DeVos’s Wednesday performance was notable. It suggests that Democrats’ best line of attack might not have been her family’s wealth and political donations or her failure to finalize an ethics agreement before her hearing. It was softball questions she should have been able to answer, not harsh attacks, that tripped her up every time.