Pew finds that landline-only surveys are biased toward Republicans

Polling techniques have become more complicated in recent years with the introduction of cell phones. In the past, researchers could reasonably assume most US residents could be accessed through a landline. However, Pew now suggests there may be a political bias in surveys that only access people though landlines:

Across three Pew Research polls conducted in fall 2010 — conducted among 5,216 likely voters, including 1,712 interviewed on cell phones — the GOP held a lead that was on average 5.1 percentage points larger in the landline sample than in the combined landline and cell phone sample…

The difference in estimates produced by landline and dual frame samples is a consequence not only of the inclusion of the cell phone-only voters who are missed by landline surveys, but also of those with both landline and cell phones — so called dual users — who are reached by cell phone. Dual users reached on their cell phone differ demographically and attitudinally from those reached on their landline phone. They are younger, more likely to be black or Hispanic, less likely to be college graduates, less conservative and more Democratic in their vote preference than dual users reached by landline…

Cell phones pose a particular challenge for getting accurate estimates of young people’s vote preferences and related political opinions and behavior. Young people are difficult to reach by landline phone, both because many have no landline and because of their lifestyles. In Pew Research Center surveys this year about twice as many interviews with people younger than age 30 are conducted by cell phone than by landline, despite the fact that Pew Research samples include twice as many landlines as cell phones.

This seems to make sense: those who have cell phones and don’t have landlines are likely to be different than those who are reached by landlines.

A few questions that I have: does this issue exist in all phone surveys today (and this article suggests there was a sizable differences between landline people and cell phone people in five of six surveys)? Have other polling firms had similar findings? If Pew now has some ideas about the extent of this issue, is the proper long-term response to call more cell phones or to weight the results more toward cell phone users?

One possible response would be to include multiple methods for more surveys. This might include samples of landline respondents, cell phone respondents, and web respondents. While this is more costly and time-consuming, research firms could then triangulate results.