Weighting and Adjustment
How pollsters correct for non-representative samples by weighting responses to match known population demographics.
Why Weighting Is Necessary
No survey sample perfectly mirrors the population. Young people are harder to reach by phone. White college graduates are more likely to participate in online panels. Politically engaged people are more likely to complete surveys. Without correction, these patterns would make every poll biased.
Weighting adjusts for these imbalances by giving more weight to underrepresented groups and less weight to overrepresented ones. If a poll's sample is 40% college graduates but the population is 33%, each college graduate's response is weighted down (multiplied by roughly 0.8) and each non-college respondent is weighted up. The goal is a sample that, after weighting, matches the population on key demographics: age, race, education, gender, and often geographic region.