In 1936, the newsmagazine Literary Digest obtained the opinions of over two million Americans regarding the presidential candidates prior to the election. Among the more than 2 million returns, Alf Landon won by an overwhelming majority over Franklin D. Roosevelt. A few weeks later, Roosevelt won the real election by a landslide. The results of the survey were not valid due to:

The selection process for the names on the list and the number of people that, in fact, answered the poll.

Explanation:

The presidential election of 1936 pitted Alfred Landon, the Republican governor of Kansas, against the incumbent President, Franklin D. Roosevelt.

For the 1936 election, the Literary Digest prediction was that Landon would get 57% of the vote against Roosevelt's 43%.

The first major problem with the poll was in the selection process for the names on the mailing list, which were taken from telephone directories, club membership lists, lists of magazine subscibers, etc.

Such a list was guaranteed to be slanted toward middle - and upper-class voters, and by default to exclude lower-income voters.

The second problem with the Literary Digest poll was that out of the 10 million people whose names were on the original mailing list, only about 2.4 million responded to the survey.