Answer
George Gallup made an accurate prediction in 1936 about Roosevelt winning by a landslide 61% of the votes. He used quota sampling to conduct his survey, using a random sample of 5000 voters. However, he failed to make a correct prediction in 1948. He proposed that Dewey, a republican, would win, which proved to be wrong and the opponent, Harry Truman, a democrat, won by 50% of the votes.
Work Step by Step
Looking back over the history of quota sampling, one can see a clear tendency to overestimate the Republican vote. In 1936, using quota sampling, Gallup predicted that the Republican candidate would get 44% of the votes, but the actual number was 38%. In 1940, the prediction was 48% but the actual vote was 45%. In 1944, the prediction was 48% but the actual vote was 46%. However, in spite of the errors, Gallup was able to predict the winner correctly in 1936, 1940, and 1944. This was merely due to luck; the spread between the candidates was large enough to cover the error. In 1948, Gallup and all the other pollsters simply ran out of luck. It was time to ditch quota sampling.
