About Pew Research Center 1

About this research 2

Weighting Polls on Party Affiliation and Past Vote 4

  Key takeaways 4

  What is weighting? 6

3 ways to correct a poll’s political balance: Party affiliation, past vote and voter file information 7

Survey panels can make political party affiliation weighting easier 11

Why many polls need to weight on party affiliation or past vote 14

  A root cause: Low response rates make surveys more vulnerable to errors 14

  Recently, pollsters have been fighting a systematic bias 15

  Another reason for weighting on party affiliation: Bouncing poll numbers 16

FAQs about political weighting in polling and its limitations 18

Acknowledgments 22