Cell phone only users are usually those under 30, a group that has been strongly supporting Barack Obama. For their samples most pollsters use a random digit dialer that call only landlines. Therefore they are missing a lot of younger voters in their polling. To make up for this most (respectable) pollsters stratify their sample, making sure they get enough voters in each subgroup including young voters (<30).
The assumption being made with this stratified studies is that the landline user group adequately represents the whole population of cell phone users. Pew Research group recently looked into this problem and found some interesting results...
"Combining polls it conducted in August and September, Pew found that of people under age 30 with only cell phones, 62 percent were Democrats and 28 percent Republicans. Among landline users the same age, that gap was narrower: 54 percent Democrats, 36 percent GOP.
Similarly, young cell users preferred Democratic presidential candidate Barack Obama over Republican nominee John McCain by 35 percentage points. For young landline users, it was a smaller 13-point Obama edge."
What could the reason be for this difference? Possibly, cell phone only users are those living in cities and more technologically enabled - two groups that seem to go strong for Obama. In epidemiology speak, age is an effect modifier in this election, and it appears Pew believes "cell phone only use" is a confounder. Nate Silver mentions "Urban voters are about 50 percent more likely to be cellphone-only than rural voters, for instance, and while some pollsters weight by geography, others do not. Thus, you may wind up with a biased sample." So this leads to the possibility that other polling outfits are slightly (unintentionally?) biased towards McCain.
So what can we infer from this?
1. Pollsters are using data from the group of young landline users, who are currently +13 Obama.
2. They are inadvertently excluding from the group that is +35 Obama
3. I have seen data stating about 50 percent or more of adults under 30 are cell phone only users (though less likely to vote than their counterparts).
4. Pollsters weigh younger voters under 30, as about 20 percent of the sample.
So according to Pew, pollsters are inputing the +13 Obama data, when they should be inputing +24 Obama data. Assuming that this data is worth 20 percent of the sample, that equals to a 2.2 percent difference in Obama's favor.
-Polls appear to be under polling Obama by a percentage point or two (estimated 2.2 points) based on this variable alone.
I'm not sure how I missed this. It's a good summary of what polls use cell phone data. Nate comes up with a similar value +2.8 Obama comapring two sets of data.