Preliminary results from the January–June 2015 National Health Interview Survey (NHIS) indicate that the number of American homes with only wireless telephones continues to grow. Nearly one-half of American homes (47.4%) had only wireless telephones (also known as cellular telephones, cell phones, or mobile phones) during the first half of 2015—an increase of 3.4 percentage points since the first half of 2014. More
than two-thirds of all adults aged 25-34 and of adults renting their homes were living in wireless-only households. This report presents the most up-to-date estimates available from the federal government concerning the size and characteristics of this population.
...At FiveThirtyEight, Nate Silver writes:
Many health surveys, political polls, and other types of research are conducted using random-digit-dial (RDD) telephone surveys. Despite operational challenges, most major survey research organizations include wireless telephone numbers when conducting RDD surveys. If they did not, the exclusion of households with only wireless telephones (along with the small proportion of households that have no telephone service) could bias results. This bias—known as coverage bias—could exist if there are differences between persons with and without landline telephones for the substantive variables of interest.
One of the biggest fears of using Internet polls is that they’re not representative of the electorate. You can weight to get rid of some disparities in a volunteer sample — if you have too few elderly respondents, for example, then the elderly respondents youdo have get more weight in the average. But weighting may not be enough; the type of older Republican who signs up for an Internet poll may differ politically from the older Republican who answers only a telephone poll. Although this isn’t a big deal in general elections in which party identification tends to determine vote choice, it’s unclear how well Internet polls that weight by party identification will do in the primary.4
A recent YouGov poll sponsored by the University of Massachusettsconfirmed these fears. UMass decided to match the YouGov likely voters to the list of active registered voters to see whether they could be confirmed as registered to vote. Among confirmed voters, Trump did 3 percentage points worse. Meanwhile, the other candidates did, on net, 10 percentage points better. Thus, even though there were fewer undecided respondents among those confirmed as registered to vote — so, all else being equal, every candidate’s vote share should have increased — Trump lost ground.
Automated phone polls, of course, can call actual voters through a list sample. They can’t, however, call cellphones (it’s the law). In order to make up for this deficiency, pollsters who perform them often use Internet panels, which themselves may not be representative.