The steady growth of publicly-available political polling in recent years has in part been due to the developing of automatic calling technologies–often known as “robocalls”–that make surveying less expensive and complicated than in the past. But this technological development has collided with another: the significant number of people, particularly younger folk, who don’t own a land-line phone.
As pollsters Paul Maslin and Jonathan Brown explain at Salon.com, many of their brethren are simply missing cell-phone-only voters, who, since they also tend to be concentrated in a particularly pro-Obama demographic, may represent a hidden margin for Obama amounting to perhaps 2 percent of the electorate.

By law, cellphone users cannot be called by an automatic dialing system (to prevent obnoxious telemarketing), and cellphone numbers are not part of the normal random-digit-dialing residential-exchange universe. Survey companies prefer to conduct polls using automatic dialing, but to find cellphone-only voters, they must employ the less-efficient hand-dialing method. Cellphone users must be sampled separately and at greater cost in time and money. This means that polls utilizing the cheaper and more efficient means of making survey calls do not include cellphone interviews.
And as survey respondents, these voters are less cooperative anyway. Even if they are contacted, they are less likely to take a call, or to arrange a call-back, than land-line households — further increasing the cost of reaching them.

Maslin and Brown dismiss substitutes for surveying of completely “wireless” voters as inadequate, and offer some evidence that this omission matters in polling results: “Gallup Poll results from earlier this year (prior to Obama’s designation as the presumptive Democratic nominee) had a 4-point swing in favor of Obama once cellphone-only respondents were folded into the overall sample.”
Most intriguingly, Maslin and Brown suggest that a similar failure to fully account for new technologies may have been an important factor in the most famous failure of the polling industry: the 1948 presidential election won by Truman against the predictions of virtually every polling operation. “Pollsters may have missed some Democratic voters in 1948 because they were technologically behind Republican voters. They were less likely than Republicans to have land lines.”
Tuck this insight away in the back of your mind for the next time you read a poll based on robocalls.