2008 Democratic Presidential Race - Iowa Polling Confusion

According to Pollster.com, two new Iowa polls show a very close race but with two different leaders. Here's what they write:

Two new polls of "likely Democratic caucus goers" conducted over the last ten days that show very different results. The American Research Group (ARG) survey (conducted 8/26-29, n=600) shows Hillary Clinton (with 28%) leading Barack Obama (23%) and John Edwards (20%). And a new survey from Time/SRBI (conducted 8/22-26, n=519, Time story, SRBI results) shows essentially the opposite, Edwards (with 29%) leading Clinton (24%) and Obama (22%).

The article goes on to complain about the lack of disclosure of methodology used in the polls, and then praises the Time poll for disclosure:

The sample source was a list of registered Democratic and Independent voters in Iowa provided by Voter Contact Services. These registered voters were screened to determine their likelihood of attending the 2008 Iowa Democratic caucuses.
Likely voters included in the sample included those who said they were
100% certain that they would attend the Iowa caucuses, OR
probably going to attend and reported that they had attended a previous Iowa caucus.
The margin of error for the entire sample is approximately +/- 5 percentage points. The margin of error is higher for subgroups. Surveys are subject to other error sources as well, including sampling coverage error, recording error, and respondent error.
Data were weighted to approximate the 2004 Iowa Democratic Caucus "Entrance Polls," conducted January 19, 2004.

Turnout in primary elections and caucuses tends to be low, with polls at this early stage generally overestimating attendance.
The sample included cell phone numbers, which, to the extent SRBI was able to identify them, were dialed manually.

I emailed Schulman to ask about the incidence and he quickly replied with a "back of the envelope" calculation: Their sample of 519 likely caucus goers represents roughly 12% of eligible adults in Iowa (details on the jump), exactly the same
percentage as obtained by the recent ABC News/Washington Post poll, but higher than the reported 2004 Democratic caucus turnout (5.5% of eligible adults). Keep in mind, however, that the ABC/Post poll used a random digit dial methodology and screened from the population of all Iowa adults.

Keep in mind that these polls make calls to landlines and not cell-phones. I decided to check on articles related to the matter of cell phones and polls, and found one by the Pew Research Center that reported only 7 percent of the population was "cell-phone only" -- but that was in 2005. A more recent study of this year now reports that estimate to be up to 16 percent, more than double the count in just two years. Thus, I argue that with such a rate of growth, the cell-phone only population will be up to about 25 percent -- one quater of the population -- by election year 2008.

The Pew report explains that the exclusion of cell phones in 2005 probably renders a poll in error by one-percent. But considering that rate and this population increase, it's reasonable to argue that the polls are inaccuate by as much as 4 percent. If you add the error term of 5 percent in the case of the Time Iowa poll, it means a whopping 9 percent error, basically making the Iowa Democratic race impossible to call.

Stay tuned for my video on this.