Polling and cell phones

Continuing on with the “poll-watching” theme, the Pew Research Center published the results of three comprehensive surveys they did after the primaries were over that included data from both land line and cell phone lines. In previous elections, cell phones weren’t widely polled, but this year more pollsters are doing that – with some starting at the beginning of the year, while others started doing so later on. Pew’s done their own major surveys on cell phone versus landline polling and the demographics of both, and here were the results:

Current polling in the 2008 presidential election shows a very tight race between Democrat Barack Obama and Republican John McCain. In part because of the strong support Obama is attracting among younger voters, and as the number of Americans who are reachable only by cell phones rises, interest continues to grow in the question of whether public opinion polls that do not include cell phones are accurately measuring the relative levels of support for the two candidates.

The Pew Research Center for the People & the Press has conducted three major election surveys with both cell phone and landline samples since the conclusion of the primaries. In each of the surveys, there were only small, and not statistically significant, differences between presidential horserace estimates based on the combined interviews and estimates based on the landline surveys only. Yet a virtually identical pattern is seen across all three surveys: In each case, including cell phone interviews resulted in slightly more support for Obama and slightly less for McCain, a consistent difference of two-to-three points in the margin.

Pollster.com’s Mark Blumenthal assesses Pew’s findings:

Pollsters have long understood that the cell phone only population — those who have cell phone but no landline telephone service — tend to be younger, and that the growth of that population has made it more difficult to reach 18-29-year olds. However, the conventional wisdom among pollsters has held that weighting by age could mostly alleviate any potential bias, as they did they did in 2004.

The new Pew report shows why weighting by age may not have the same effect now:

[…]

The roughly two-to-three point difference in the margin favoring Obama is, as it happens, very close to the effect Nate Silver obtained over the weekend by comparing results from pollsters that have been interviewing by cell phone (including Pew) with a control group that has not.

So does this mean that you can look at all the polling that’s been done over the year and add 2 to 3 percentage points to Obama’s totals already and subtract 2 to 3 from McCain’s? It’s hard to tell, because you have to take into account who did the polling, when it was done, the margin of error, and a lot of other factors. But let’s assume for purposes of discussion that you do add to Obama’s totals and subtract from McCain’s. As per Pew’s findings, a large percentage of those polled via cell phone are young voters, historically an unreliable voting bloc. Fickle young voters could make or break the election for the O-man.

We shall see …