In advance of the 40th anniversary of the Supreme Court’s Roe v. Wade decision, the Pew Research Center released a poll on Wednesday finding that 63% of Americans oppose completely overturning the ruling, which created a constitutional right to abortion and legalized most abortions nationally.

Just 29% of Americans want to see the ruling overturned, according to Pew, which added, “These opinions are little changed from surveys conducted 10 and 20 years ago.”

These are the headlines. But dig a little deeper and it becomes clear that the poll tells us little about Americans’ views on the legality of abortion.

According to Pew, only 62% of Americans know that Roe dealt with abortion. Indeed, 7% of respondents thought Roe dealt with school desegregation. (And I think it’s safe to assume most of them oppose overturning Roe.) Only 44% of respondents 30 years old or younger correctly identified abortion as the issue dealt with in the case.

Interestingly, self-identified Republicans were 11 percentage points more likely than Democrats (68% to 57%) to know that Roe addressed abortion.

There’s some good news in the poll for pro-life advocates: 74%of those who believe Roe should be overturned think abortion is either a “critical issue” or “one of many” critical issues, while just 31% of respondents who don’t want Roe overturned believe it’s critical. This underscores the “intensity gap” that pro-choice advocates have long lamented.

And there’s something to hearten Republicans: Pew found that 41% of respondents felt Democrats can do a better job of representing their views on abortion, while 36% felt Republicans would do so. Last March, Democrats held a 16-point advantage on that question. But again, all these data should be taken with a grain of salt.

Here’s another problem with the poll. Pew didn’t phrase the question very well. It asks,

Would you like to see the Supreme Court:

A) Completely overturn Roe v. Wade

B) Not overturn Roe v. Wade

This leaves no room for those who may believe that the ruling should be partially overturned.

Opinion polls about Roe v. Wade tell us little that’s meaningful. That’s not only because a significant minority of Americans can even link Roe with abortion. Many others cannot get to the next level of understanding by correctly identifying how Roe affects the availability of abortion.

In a 2007 study of registered voters, 55% of respondents said they didn’t want Roe overturned, while 34% said they wanted it overturned.

But then respondents were informed that Roe prohibits states from limiting abortion during the first six months of pregnancy, and that without Roe states could enact laws allowing abortion in some instances and prohibiting it in others.

With this knowledge, support for upholding Roe sank 7 points, and support for overturning Roe rose 9 points, a 16-point swing.

Decades of abortion polls make clear that many Americans simply consider Roe to be a synonym for the right to abortion, and that without Roe all abortions would be illegal. Consider that the 29% of Pew respondents who want to see Roe overturned is not all that much higher than the 20% of Americans who tell pollsters that abortion should be illegal under all circumstances.

Under Roe, most abortions are legal, and polls suggest roughly 25% of Americans support abortion under any circumstances.

Add the 9% difference from above to this 25% and you get 34%. Who are these 34%? They’re the ones we know actually have a clue about what Roe means.

Opinion polls about Roe v. Wade are supposed to shed light on how America feels about one of the Supreme Court’s most controversial rulings. But all they really tell us is that most Americans don’t know what Roe said or how it affects the right to abortion.