Nate Cohn look at the faults of “so-called online opt-in panels” and why these polls tend to fare poorly despite their popularity.
“Nonetheless, a majority of polls are now conducted in exactly this way: fielded online using people who joined (that is, opted into) a panel to take a survey, whether by clicking on a banner ad or via an app.
Traditional polling, in contrast, attempts to take a random sample of the population, whether by calling random phone numbers or mailing random addresses.”