Let's consider the 2010 senate pre-election polls. Based on 37 LV polls (the GOP led the average by 48.1-43.5%) , t he pre-election model predicted a 50 -48 Democratic Senate.
CNN/Time provided RV and LV polling data for 18 Senate races (Table 1). The Democrats led a combination of 18 RV and 19 LV polls by 45.2-44.6% giving them a 53-45 seat majority .
The Democratic RV margin was approximately 5% higher than the LV margin.
RV polls were not listed in the final realclearpolitics.com polling averages. The Democrats led the average RV poll by 49.2-40.6%. They also led the corresponding LV s by 46.6-45.8%.
The Democratic margin was 8% higher in the RV polls.
The 2010 Final National Exit Poll indicated that 45% of the electorate were returning Obama voters and 45% were McCain voters. Obama's recorded vote margin was 52.9-45.6%. Of course, the pundits will claim that the 7.3% discrepancy was due to millions of unenthusiastic Democrats who did not return to vote in 2010. The pollsters make adjustments to the number of returning voters (the "mix") and the vote shares in order to match the vote count. Obama won the recorded vote by 9.5 million, but his True Vote margin was at least twice that. His recorded share understated his True vote share by 4-5%. If the 2010 NEP returning voter mix is adjusted to match the 2008 recorded share (53-45%), the average Democratic share is within 1% of the GOP share -- and matches the pre-election RV polls. The adjusted 53% Democratic share of the 2010 electorate is 5% lower than Obama's True share.
Table 2 is a comparison of 18 pre-election Registered (RV) and Likely Voter (LV) polls and the recorded vote. Democrats led the RV poll average by a solid 46.4-41.1%. They led the LV sub-sample by 46.6-45.8%, within 0.4% of the recorded 48.3-47.9% share.
Table 3 is a comparison between the final pre-election LV polls, the preliminary exit polls and the recorded shares. There was a 2.9% discrepancy in margin between the average recorded vote and the exit poll. The final RCP LV projected average margin exceeded the recorded margin by 2.2%.
Table 4 displays Final exit polls Gender crosstab.
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