Scientific polls can be conducted by phone, via online panels or some other way, and use a mix of sampling and weighting to make their numbers representative of the larger population whose opinions they’re measuring ― whether that’s all adult Americans, or just likely voters. Recent changes in technology have complicated that process, but the underlying principle remains basically the same.

That’s why, even if a scientific poll reaches relatively few people, it can accurately depict the opinions of a much larger group.

In contrast, reader polls, like those Trump cites...make no such attempt to weight their responses or to represent anything beyond the number of people who happen to have clicked on them...And that’s without getting into what’s possibly the worst problem with reader polls ― there’s often nothing to prevent people from voting multiple times.That leaves them vulnerable to intentional manipulation by people with a vested interest in the outcome ― a fact that some Trump fans took full advantage of.