… People usually don’t know why they vote for the candidates they choose to vote for, and are not particularly good at assessing how something influenced that vote—let alone how some hypothetical future event would influence them.
...if you ask voters, it turns out that some will tell you that they would be more likely, and a somewhat larger number will tell you that they’ll be less likely, to vote for someone with a Trump endorsement. Hey, reporters: don’t believe those polls! You can take it as a measure of what respondents think about Trump, if you care about such things, but there’s no reason to believe that this kind of self-reporting about vote choice is meaningful at all, and it shouldn’t be included in stories about a Trump endorsement as if it was meaningful.
...The bottom line here is that polling is a really good tool for reporters to use in many cases, but remember: what polling tells you for sure is only what people will say if they’re asked a question by a pollster.
Jonathan Bernstein
Working in market research, I have to resist the impulse to point this out practically every day.