Ottawa Citizen: Why you can trust the census, but not polls

[tuvor]

In 1936, a massive poll of 2.3 million Americans revealed that the forthcoming presidential election would be won in a landslide by Alf Landon. Alf, who? Right. There was indeed a landslide in 1936, but the winner was Franklin Delano Roosevelt.

Yes, this is ancient history. But bear with me. This famous poll is a standard illustration in Stats 101 classes. With the census controversy continuing to dominate the front pages of newspapers, we could all use a little Stats 101.

It's hard to overstate just how mammoth that famous poll was, particularly in an era before computers. Conducted by The Literary Digest, a popular magazine, the first step was to gather an astonishing 10 million addresses from automobile registration lists and telephone books. Ballots were mailed to every one of those addresses. Roughly 2.3 million were returned. When the numbers were added up, it was Landon in a landslide. And with a survey that big, how could it be wrong?

And yet it was wrong. For two reasons.

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http://www.ottawacitizen.com/opinion/trust+census+polls/3330368/story.html