Is it safe to past vote weight?

1. Is it safe to past vote weight? John Curtice

2. Why past vote weight? • Principal aim of opinion polls is to secure accurate estimates of vote intention – but have exhibited bias to Lab since 1987. • Demographics are nowadays relatively weakly related to vote, and weighting thereby cannot be relied upon to ensure accurate estimates • Vote at last election (‘past vote’) is strongly related to current vote intention and distribution is known • So makes sense to past vote weight!

3. However… • People may forget or misremembered how they voted last time (‘recall error’) • And may do so non-randomly • In particular, may be inclined to avoid cognitive dissonance by aligning past vote with current preference • And to forget more ‘esoteric’ choices • In which case past vote weighting could simply make a good poll bad.

4. Himmelweit, Biberian and Stockdale • British Journal of Political Science 1978 • Longitudinal study of young men 1962-74 (N=450) • Asked after each election how voted – and in 1970 (N=365) and October 1974 (N=246) asked to recall vote at two previous elections. • Results of recall compared with reported vote immediately after election.

5. Butler and Stokes • 1963-64-66-70 panel study. • Asked how voted immediately after each election • And in 1970 also asked to recall 1964 and 1966 vote • N=846, and originally demographically representative

6. Accuracy of Recall

7. Aligning with current vote

8. Some votes harder to remember!

9. Perhaps? • People less likely to align recall vote with vote intention than with reported vote at the last election? • Not least because less time elapsed • Liberal Democrat no longer an ‘esoteric choice’ – more votes and stronger organisational presence.

10. British Election Panel Studies • Followed panel of voters through 1992-7 (N=1,347) and 1997-2001 (N=1,594) parliaments • Reported vote obtained in weeks after election • Recall vote asked annually thereafter, including after next election

11. Accuracy of Recall

12. Aligning with current vote intention

13. What Himmelweit did not show

14. Did I really vote Lib Dem? - 1

15. Did I really vote Lib Dem? - 2

16. But consider… • Assume Con got 40% at last election; Lib Dem 20%. • Say 10% of those who voted Lib Dem last time now mistakenly claim that they voted Conservative. • Only requires 5% of those who voted Con last time to claim mistakenly they voted Lib Dem to balance the error. • So more forgetful Lib Dems ≠ fewer Lib Dems

17. The totals compared - 1

18. The totals compared - 2

19. Conclusion - Aligning • Those who misremember do have a tendency to align their recall vote with their current vote intention • But most people do not misremember • And most who have changed their allegiance since the last election do not align their recall vote with their current preference

20. Conclusion – ‘Esoteric’ Choices • People are still more likely to forget voting Lib Dem (or abstaining or voting for any ‘other) than Con or Lab • But because they represent a smaller poll of voters, they may still not be underrepresented in distribution of recall vote • So past vote weighting much ‘safer’ than Himmelweit et al suggested

21. However… • Past vote weighting cannot deal with any bias amongst those who do not have a past vote • The impact of recall error apparently varies – across parties and parliaments • So not necessarily adequately taken into account by applying a constant ‘discount’ factor - as is current common practice • Need continuously updated panel – like BEPS. A role for Understanding Society?