Since 1989, Gallup has conducted most of its nationwide U.S. surveys by telephone, using random-digit-dialing (RDD) methodology. Gallup’s telephone polls were initially conducted in the continental U.S. only, but Hawaii and Alaska were added to the sampling frame in 2008. Cellphones were also introduced in 2008 and have become an increasingly large proportion of the samples, conforming with government estimates of the cellphone-only population among U.S. adults. The typical sample size for these surveys has been 1,000 national adults, aged 18 and older. The margin of sampling error is specific to each survey but has generally been ±3 or ±4 percentage points.
Before 1989, Gallup conducted its national polls primarily through face-to-face interviews in the continental U.S. with approximately 1,500 adults. These had a margin of sampling error of ±3 percentage points. From 1935 to 1970, the polls were conducted with adults aged 21 and older; in 1971, after the voting age was lowered to 18, Gallup expanded the sampling frame to include 18- to 20-year-olds.