The political world’s perception of Tuesday’s Republican primary for U.S. Senate in West Virginia is largely being shaped by a pair of stories — one in Politico, the other in the Weekly Standard — claiming that Don Blankenship, the coal magnate who spent a year in prison for safety violations relating to the Upper Big Branch Mine disaster in 2010, is surging in the race’s final days. That’s a problem: Both stories relied on that most dangerous of sources, the biggest tease in politics: the internal poll.

Internal polls — politico-speak for polls conducted by a campaign or another entity with a stake in the race — have their uses. Indeed, in this case, we don’t have any recent public polls of the race. But you should take internal polls with several grains of salt. (FiveThirtyEight editor-in-chief Nate Silver would recommend a whole truckload of Morton.) Specifically, it’s important to remember that by definition, internal polls are shared by people with an agenda, and you need to take that agenda into account when looking at the results. In the West Virginia case, the agenda appears to be to marshal the non-Blankenship vote, but I’ll get to that in a moment.

Usually, the goal in leaking an internal poll is simple: make your candidate look good. In practice, that means that internal polls typically paint a rosier picture for the candidate conducting them than exists in reality.