- Given the compressed caucus and primary calendar, polls conducted before the New Hampshire primary may have ended too early to capture late shifts in the electorate’s preferences there.
- Most commercial polling firms conducted interviews on the first or second call, but respondents who required more effort to contact were more likely to support Senator Clinton. Instead of continuing to call their initial samples to reach these hard‐to‐contact people, pollsters typically added new households to the sample, skewing the results toward the opinions of those who were easy to reach on the phone, and who more typically supported Senator Obama.
- Non‐response patterns, identified by comparing characteristics of the pre‐election samples with the exit poll samples, suggest that some groups who supported Senator Clinton—uch as union members and those with less education—ere under‐ represented in pre‐election polls, possibly because they were more difficult to reach.
- Variations in likely voter models could explain some of the estimation problems in individual polls. Application of the Gallup likely voter model, for example, produced a larger error than was present in the unadjusted data.
After Clinton's concession in June 2008, Democrats were more generous toward supporters of their own preferred candidate than to supporters of the other Democratic candidate. The bias observed in June persisted into August, and disappeared only in early September after the Democratic National Convention. We also observe a strong gender effect, with bias both appearing and subsiding among men only.