What did the selected poll say?
Public Policy Polling’s selected estimate put Mark Warner at 49.0% and Ed Gillespie at 40.0%. On the chart’s stated basis, that was D +9.0. The corresponding recorded outcome margin was D +0.8, for a 8.2-point difference in the margin.
What actually happened?
The selected PPP poll put Warner at 49 and Gillespie at 40; Warner finished near 49.15 while Gillespie finished near 48.34. The published margin missed by about 8.2 points even though the poll's leader won.
Warner's estimated share changed little in the comparison; the numerical miss is concentrated in Gillespie's share. That does not prove that particular undecided or Sarvis respondents switched to Gillespie.
Why might the poll have missed?
No race-specific methodological audit establishing a single cause was verified for this case. Possible turnout, nonresponse and late-choice mechanisms remain possibilities. A numerical difference between candidate shares does not track individual voter movement.
What has not been established?
No race-specific methodological audit establishing a single cause was verified for this case. Possible turnout, nonresponse and late-choice mechanisms remain possibilities. A numerical difference between candidate shares does not track individual voter movement.
How should we use this example?
Look at multiple independently documented polls on the same dates, while retaining their uncertainty and sample definitions. This page uses one transparent example; it does not infer that every Virginia poll made the same mistake.
One PPP likely-voter poll is compared with the current Virginia Department of Elections total, including other candidates and write-ins. Its reported margin miss is not an industry-wide polling error.
What to carry forward
A nearly correct estimate of the winner's share can still produce a badly wrong margin.