How biased are polls?

Lydia Saad, a senior editor at Gallup, argues that if a pollster tries to control what percentage of those polled belong to the respective parties, the pollster might well be predetermining the results. “In a wave election, let’s say,” Saad explains, “there is a huge shift of voters toward a certain candidate. In that case, you’re going to see that party’s ID go up in the polls. Does that mean you push it back down? Well, no, because you might as well just decide what percentage of the vote each candidate is going to get and don’t even bother polling.”…

Pollster Scott Rasmussen concurs. “It does not mean it’s a bad poll,” he says, “if it has an oversampling of Democrats or an oversampling of Republicans or something else that conflicts with a perception that a person might have, but it is something that you should take into consideration in evaluating, especially two polls from the same company. If a company comes out with a poll one day that shows the president leading by five points, and the next day it has a poll that shows the race tied, and you look and see the only thing that changed was that the partisan sample changed, that’s probably just noise as opposed to real significance.”