There has been a lot of discussion of the Pew Report released earlier this week that shows that including cell-phone only respondents does appear to make a 2-3% difference in the presidential preference polling (see Mark"s post). What's most intriguing to me is how this would play out at the state level. Indeed, it seems very unlikely that every state has the same percentage of cell phone only households. Thus, in states with fewer cell-phone only users, the effect of excluding such respondents may have less of an effect on the poll results. On the other hand, states where there are more cell-phone only households may have polling that is further off the mark.
I was hoping to be able to easily find some survey data with enough respondents to get a sense of the prevalence of cell-phone only households in each state. Unfortunately, the 2007 CDC data that is often cited provides more than enough national interviews to accomplish this task, but the dataset hides the state of the respondent, only allowing users to place respondents in a particular Census Region. Nevertheless, we can learn a little about geographical variance from this data. Specifically, families in the South and Midwest are more likely to have cell phones only compared to states in the West and Northeast. Based on this data, we should expect polling from southern and midwestern states to be more prone to error from the cell-phone only problem than polling in other regions.
Another source of data that may be of some use in answering this question is the 2006 Cooperative Congressional Election Study. (In the interests of disclosure, I should say that I am involved in the 2008 version of this study, though I had no role in the 2006 version.) This was a large (approximately 30,000 respondents) internet survey conducted by YouGov/Polimetrix using a matched random sample design. Because this is an internet survey, it probably isn't as ideal for addressing this question as the CDC survey would be. However, the sample was stratified to assure that there would be a large enough sample from each state and since the state of the respondent is available for this data (and isn't for the CDC data), it is the one decent way I've found of breaking out cell-phone only figures by state.
In this survey, 10.6% of respondents indicated that they only had a cell-phone (this is smaller than the percentage cited in the CDC survey, though the CDC survey was conducted a year later). Most interesting is the variation across states. The map below shows this variance.
Some of the swing states that stand out as having higher than average cell-phone only users are Montana (21%), Oregon (17%), Virginia (15.7%), Wisconsin (15.3%) and Minnesota (15.1%). (Keep in mind that these figures are from two years ago). If this survey is providing reasonably accurate figures on cell-phone only users, then it may be the case that polling of these states would be particularly prone to under-stating support for Obama. It may also explain why the polling in some of these states (for example, Virginia) has been so erratic.
Of course, these data may be problematic and should be taken with some caution. If anyone has ideas about other sources that could be used to compile state-by-state measures of cell-phone only households, please let me know. If excluding cell-phone only respondents does matter, then it would be nice to have a strong sense of where it will matter most.