WASHINGTON — The quality of advocacy at the Supreme Court these days is quite high. “We have an extraordinary group of lawyers who appear very regularly before us,” Justice Elena Kagan said in 2014 at a Justice Department event.

But there was, she said, one exception. “Case in and case out,” she said, “the category of litigant who is not getting great representation at the Supreme Court are criminal defendants.”

That impression, widely shared by people who frequently attend Supreme Court arguments, has now been confirmed by a comprehensive look at a decade of data.

“Criminal defendants are almost never represented by expert counsel in arguments before the Supreme Court,” Andrew Manuel Crespo, a law professor at Harvard, wrote in the new study, which was published in The Minnesota Law Review.