Even though telephone polls start off with a random set of numbers to dial, low response rates mean that the samples pollsters end up with are often far from representative — typically younger and less-educated voters are very thin on the ground.

Michael Ramlet, Morning Consult’s co-founder and CEO, told BuzzFeed News that the idea that any pollster is today working with a true random sample is “essentially a myth.”

To correct any departures from known demographics, pollsters have long “weighted” their results. If a sample contains only half as many 18 to 25 year olds as it should, for example, and twice as many people over 65, the young voters would be given a weight of two, and the older ones a weight of 0.5. So every young person who said they intended to vote for Trump or Clinton would count twice, while two older supporters would be needed to register a single preference for each candidate.

If weighting factors become extreme, a small number of responses from an underrepresented group can skew the overall poll result. And as telephone poll response rates have dwindled, the weighting factors used by pollsters have grown. Langer Research Associates, for example, which runs telephone polls for ABC News, allows weights between 0.2 and 6.