The traditional gold standard of polling is probability sampling, where you contact people selected at random from a list of the population. But probability sampling isn’t so great anymore. With response rates in the 10 percent range, there is concern that the select group of people who happen to respond to surveys are nothing like a random sample of the population of adult Americans, or even the population of voters. …
An alternative approach is opt-in polling, often performed on the Internet. …
The question then arises, do non-representative polls work, in the sense of giving reasonable estimates after adjustments? Our answer is yes, at least for the task of tracking election campaigns, a convenient example because we can compare our estimates to a large set of existing polls and also the actual election outcome. CONT.
Andrew Gelman (Columbia), The Monkey Cage