Most Survey Items Differ Little Between Full Sample and Web Sample
This comparison of hundreds of individual survey questions, with and without the offline population, found that for most of the topics studied by Pew Research Center, the size of the bias caused by excluding the non-Web respondents is quite small. Across 406 separate survey items, only nine yielded estimates that differed by 5 or more percentage points. More than two-thirds of the 406 items yielded estimates with a 0 or 1 point difference. This is a consequence of the fact that the non-Web survey group is a small share of the total population, as well as the fact that the size of the differences between the Web and non-Web groups tends to be relatively modest on most questions.
Most Survey Items Differ Little Between Full Sample  | and Web Sample  | 180 165  | 150  | 114  | 120  | 90  | 60  | 32  | 30  | 12  | 1 3 O 2 O 2 O  | 1 2 3 4 5 6 7 8 9 10 11 12 13  | % point difference between survey estimate based  | on full sample and Web sample  | Source: 406 survey estimates from the American Trends Panel Waves3-11 conducted  | during 2014 survey and 2015  | PEW RESEARCH CENTER 