Brian Schaffner | October 23, 2008
One can get lost in the deluge of polls which, just this week, show anything from a narrow 1% Obama lead (AP-Gfk) to a substantial margin of 14% (Pew). One pattern that seems to have become particularly evident this week is that the polls showing the biggest leads for Obama tend to be those that are polling the cell phone only population (such as Pew, CBS/New York Times, and ABC/Washington Post). We know from the recent Pew report that excluding cell phone only respondents from the sampling frame reduces Obama's margin by 2-3%, even when the sample is weighted. But how does this affect the national trend estimate, which takes into account all polling?
One of the great features of the new interactive tracking charts available on this site is the ability to select or remove particular pollsters. I used this feature to create two national trend estimates--one including only pollsters that include cell phone only respondents, and one including all other pollsters.
National Trend Estimate for Pollsters Reaching Cell Phone Only Respondents
National Trend Estimate for Pollsters not Reaching Cell phone Only Respondents
The comparison between the two trends is remarkably consistent with what the Pew Report would lead us to expect. While the trend that includes pollsters not calling cell phones shows an Obama advantage in the 6-7% range, the trend for those reaching cell phone only respondents shows an Obama lead greater than 10%. Obama's support increases by almost 3% in the national trend that includes polls reaching cell phone only respondents while McCain's support decreases by about 1%.
The difference between these two trends is hardly trivial since an extra 3-4% in the national vote could very well mean that several additional states tip in Obama's favor, producing a substantial electoral college landslide (keep in mind that most statewide polls are not including cell phone only respondents). If we assume that polls that reach the cell phone only population are more accurate, then Obama's lead may very well be in double-digits. But on November 4th, it will be worth checking back on these two trends to see whether the cell phone only pollsters actually do fare better in predicting the election outcome.
UPDATE: One of the nice things about the dynamic charts is that they will continue to update themselves. Thus, if you want to keep track of the differences between the separate trends for the next 11 days, you can bookmark this post and keep checking in.