The 1936 Presidential Poll Disaster
Literary Digest magazine's massive survey:
- Sent 10 million survey forms
- Received 2.4 million responses (largest poll in history)
- Confidently predicted: Alf Landon wins by landslide
Actual election result: Franklin D. Roosevelt won by actual landslide (60.8% of vote)
What went wrong? They surveyed people from car registrations and phone directories. In 1936, only wealthy people owned cars and phones. Wealthy people voted Republican. The majority (poor/middle class) voted Democrat but weren't in the sample.
The lesson: Sample size doesn't matter if your sample is biased.**