Donald Trump insists he won the presidential debate on Monday night. And to prove it, he’s pointing to the polls.

Hillary Clinton also insists she won the debate. And to prove it, she too is pointing to the polls.

What’s going on here?

The key difference is the kind of poll the candidates are citing. Trump is going on unscientific online polls, which don’t have any controls to make sure they are actually representative of American voters. Clinton’s claim relies on more rigorous scientific polls — those like CNN’s — that apply controls to try to be representative of US voters.

Because of that, Clinton’s claim that she won the debate is much more credible. As Huffington Post polling director Ariel Edwards-Levy put it on Twitter, “A poll that does not exert some measure of control over who takes it and how many times they do so is not a poll and you should ignore it.”

The difference between unscientific and scientific polls

The polls that Trump is relying on let anyone vote with absolutely zero checks. If you’re online at the time and find the poll, you can vote. You don’t have to live in America or be a US citizen. And you can vote multiple times — by reopening a browser tab, going behind an internet proxy, or logging on to a different account.

As an example, you could right now log on to different Twitter accounts to spam down the results of USA Today’s poll for whichever candidate you prefer:

Who do you think won last night's #debate? https://t.co/rlKNUH4jdr — USA TODAY (@USATODAY) September 27, 2016

This can lead to some very skewed results. For example, if an active online community — like r/The_Donald, the Reddit community that supports Trump — gets a bunch of people to vote on a poll (as they did), this can lead to Trump supporters overwhelming the results with a higher percent of Trump supporters than would otherwise be present in a typical sample of American voters. With such a skewed sample, it’s impossible to take the results seriously — it turns into a contest over which online community is most enthusiastic about winning unscientific polls, not how US voters feel about who won the debate.

This is how Trump came ahead in a few online polls, including those at Drudge Report, Time magazine, and CNBC.

The polls Clinton is relying on, on the other hand, use statistical controls to make sure the sample isn’t so skewed. They try to contact people that match the voting population — so they’ll try to ensure that a certain percent of respondents in the survey are white, black, Latino, Democrat, Republican, and so on. And if they can’t reach the right amount of people, they’ll sometimes adopt statistical weights to bring up or down a specific group — so if a survey has too many men, they might try to weigh the women’s responses higher.

The CNN/ORC poll on Monday night was a scientific one. CNN acknowledged its sample of Democrats was a bit too high — since this was a poll taken quickly after the debate, the network and its pollster just didn’t have time to do better. Still, the win was overwhelmingly for Clinton — with 62 percent of voters who watched the debate saying that she won versus 27 percent saying the same for Trump.

CNN’s poll wasn’t the only scientific one to reach this conclusion. Public Policy Polling’s post-debate poll found that 51 percent of debate watchers said Clinton won, versus 40 percent who said the same of Trump. So far, CNN and PPP’s polls are the only two scientific polls we have.

There are questions about how methodologically rigorous even these scientific polls are, given that they’re done so quickly after the debate. But Nate Silver pointed out on FiveThirtyEight that scientific post-debate polls like CNN’s tend to correlate with the longer-term trend in broader polls after debates. There’s no reason to think that an unscientific poll would have the same value.

Still, it’s impossible to say what the ultimate effect of the debate will be. As Andrew Prokop explained for Vox, debates tend to have a small impact — but in a very close election, this small impact can be decisive. So we’ll just have to wait for the next few weeks of polling to see if the first presidential debate really makes a difference.

Watch: This election isn’t just Democrat vs. Republican. It’s normal vs. abnormal.