A biased sample is one that is falsely taken to be typical of a population from which it is drawn. Someone saying "Everyone liked that movie!" might not mention that the "everyone" was them and three of their friends, or a group of the star's fans.
Online and call-in polls are particularly at risk of this error, because the respondents are self-selected. At best, this means the people who care most about an issue will answer; at worst, people listening to a particular radio host, or on a political mailing list, flood the poll.
Biased samples are not always an attempt to mislead: in 1936, in the early days of opinion polling, the American Literary Digest magazine called two million random telephone numbers, questioned the people who answered, and predicted the election result. They got it wrong because, at the time, telephones were far from universal, and telephone owners were not a good sample of the electorate as a whole. In contrast, a poll of only 50,000 citizens selected by George Gallup's organisation successfully predicted the result, leading to the popularity of the Gallup poll. 1936 was a leap year starting on Wednesday (link will take you to calendar). ...
The Literary Digest was an influential general-interest magazine in the early 20th century United States. ...
George Horace Gallup (November 18, 1901 - July 26, 1984), American statistician, invented the Gallup poll, a successful statistical method of survey sampling for measuring public opinion. ...
A Gallup Poll is an opinion poll frequently used by the mass media for representing public opinion. ...
- According to a survey of delegates at the Communist Party Convention, the Communist Party is the most popular political party in the country.
A special case of this is the spotlight fallacy. This is the fallacy of assuming that all of a group correspond to those members that receive most attention, from the media or otherwise.
- I wouldn't like to go to America because the of all the gun crime, we see it on the news all the time.
- Doctor: Why don't patients make some effort to look after themselves? My surgery is full of people who eat, drink, smoke and don't get any exercise. Of course he may have many more patients who do look after themselves and don't often turn up in his surgery.
- Why do young people all take drugs and go around mugging old ladies? You read about it in the paper all the time!
- Child: When I grow up I want to be a singer. Have you seen how much money those pop-stars make?!