'Missing' responses

The first two types of non-sampling error are caused by failure to obtain information from some members of the target population.

Coverage error

Literary Digest Poll

Incidentally, another pollster, George Gallup, also conducted a survey before this election. Although he only sampled 50,000 people, he put more effort into making his sample representative. His poll predicted that Roosevelt would win the election with 56 percent of the vote, much closer to the actual result.