As the returns come in, many media outlets will begin sharing exit poll data.

Studies of the last four presidential elections have shown these exit polls to be tremendously skewed towards the Democrat -- meaning that they showed a much greater percentage of the vote going to the Democrat than the actual results.

With this in mind, NewsBusters readers are warned to take them with a grain of salt.

Below the fold is a memo concerning this matter from the polling organization Public Opinion Strategies:

As we have seen in previous election cycles, the exit poll results do leak early and that ends up influencing the coverage of the race before even the first state polls close at 6:00 PM Eastern. [...]

Here are the key points to keep in mind when the exit poll data starts being leaked:

1. Historically, exit polls have tended to overstate the Democratic vote.

2. The exit polls are likely to overstate the Obama vote because Obama voters are more likely to participate in the exit poll.

3. The exit polls have tended to skew most Democratic in years where there is high turnout and high vote interest like in 1992 and 2004.

4. It is not just the national exit poll that skews Democratic, but each of the state exit polls also suffers from the same Democratic leanings.

5. The results of the exit polls are also influenced by the demographics of the voters who conduct the exit polls.

After the 2004 election, the National Election Pool completed a study investigating why the exit polls that year showed John Kerry over performing 5.5 net points[1] better than the actual results showed him to have done. Their conclusion was that the primary reason the exit polls was that Kerry voters and Democrats were more likely to participate in the exit polls.

"Our investigation of the differences between the exit poll estimates and the actual vote count point to one primary reason: in a number of precincts a higher than average Within Precinct Error most likely due to Kerry voters participating in the exit polls at a higher rate than Bush voters. There has been partisan overstatements in previous elections, more often overstating the Democrat, but occasionally overstating the Republican."[2] (Emphasis added)

We believe that this will hold true this year. The recent Fox News survey[3] showed that 46% of Obama voters said they were very likely to participate in the exit polls, while just 35% of McCain supporters are.

In fact, even the 2004 exit poll report noted that higher turnout nationally and higher levels of voter interest in both 1992 and 2004 correlated with greater Within Precinct Error.[4]

The overstating of the Democratic vote did not only occur in the national exit polls, but also occurred in the state exit polls. The 2004 exit poll report cited that the Kerry vote was overstated by more than one standard error in 26 states, while the Bush vote was overstated in just four states.[5] So we should also expect the individual state exit polls on Tuesday to be more Democratic as well.

So given that turnout is expected to be even higher than 2004 and that Democrats are more likely to participate in the exit polls, this means we should expect greater fluctuation and variation in the exit polls from the actual election results.

The 2004 exit poll report also showed that the greatest error in the exit poll came in precincts where the interviewer was younger. The completion rates were lower and the refusal rates and Within Precinct Error was higher when the interviewers were under the age of 35.[6] Complicating this is that nearly half the interviewers were under the age of 35, including 35% who were 18-24 and another 15% were 25-34.[7]

Conclusions

Based on the previous exit poll results, we should expect once again that Tuesday's exit poll data could overstate the Obama vote and under represent the McCain vote.

[1] Evaluation of Edison/Mitofsky Election System, Page 20

[2] Evaluation of Edison/Mitofsky Election System, Page 3

[3] http://www.foxnews.com/projects/pdf/103008_poll.pdf

[4] Evaluation of Edison/Mitofsky Election System, Page 35

[5] Evaluation of Edison/Mitofsky Election System, Page 3

[6] Evaluation of Edison/Mitofsky Election System, Page 43

[7] Evaluation of Edison/Mitofsky Election System, Page 52

Something to bear in mind is that given the race element this year, exit polls could be even more skewed towards the Democrat than usual. 

To give you an idea why, just imagine how difficult it might have been for exit pollsters to find folks willing to admit they voted for McCain with these two Black Panthers standing by the front door of the polling station at 1221 Fairmount Avenue in Philadelphia:

—Noel Sheppard is the Associate Editor of NewsBusters. Follow him at Facebook and Twitter.