“Why does President Trump ignore facts and experts?” CNN asked in a panel discussion last fall hosted by Jake Tapper. Mr. Trump’s media critics often accuse him of ignoring inconvenient facts, while they grant academics and other scholars the status of arbiters of truth and falsehood.

If we are to play that role, we should take it seriously. So should those who report on our findings. Unfortunately, the media’s antipathy towards Mr. Trump has caused bias to infect social-science reporting. An egregious example is coverage of a study that, according to NBC News, finds “Russian propaganda may really have helped Trump.” According to the statistical analysis, written by University of Tennessee-Knoxville researcher Damian Ruck and others, Mr. Trump did better in the polls in periods after accounts connected to the Russian-linked Internet Research Agency were more active and received more engagements on Twitter.

The adage “correlation does not equal causation” is a cardinal rule among social scientists. When NBC wrote up the results of the Ruck study, it claimed that the results “remained true even when controlling for Mr. Trump’s own Twitter activity and other variables.” In fact, the study took account of no other variables. Nor did it consider an obvious alternative explanation for the correlation: that the IRA-linked accounts were particularly active during times when the news for Mr. Trump was good, and the polls that followed simply reflected such events.

In fact, in the yet-unpublished appendix, the authors find that when Mr. Trump’s own retweets are accounted for, their main results lose statistical significance. A closer look at the data shows why. For instance, IRA-linked accounts had high level of engagement during the Republican National Convention, an event that typically gives the nominee a bump in the polls. Based on such data, the authors conclude that “25,000 retweets per week over all IRA tweets . . . predicted approximately one percent increase in Donald Trump’s poll numbers.” Twitter reports that more than a billion tweets about the campaign were sent in the months before the election.

The authors of the study acknowledge in their peer-reviewed article that their data don’t support any strong claims for causation. Mr. Ruck, however, has been far less cautious, claiming in a piece for the Conversation that because the 2016 election was decided by fewer than 75,000 votes, the efforts of Russian trolls might have been enough “to affect the outcome of the election.” By that reasoning, if the election was close enough, anything that a foreign adversary did, no matter how small, could be said to have determined the outcome, giving rival states an easy way to discredit American democracy.