With the upcoming US presidential elections I shall be wheeling out stuff about opinion polling next semester. At the most basic it’s worth just asking students to think about how national polls can be accurate when they only survey around 1000 people. Students can usual derive the idea of representative samples for themselves without much prompting.
I then tell them the story of the 1936 US Election, and the spectacularly inaccurate poll conducted by ‘Literary Digest’ magazine that I first read about in the excellent book ’Critical Thinking about Research’ . What makes this story of particular interest is that the magazine polled 2 million people, and that their polls in the previous four US Presidential campaigns had been very precise. The 1936 ‘Literary Digest’ poll predicted a win for the Republican candidate Alf Landon, but the actual result was a landslide win for the Democratic candidate Franklin Roosevelt, with Landon only winning two states.
Interestingly, when I ask students why they think it might have been this wrong they seem to initially think about political bias from the publisher, however, with a little prompting about what was going on in America at the time (i.e. the Great Depression) they get around to asking who the magazine actaully questioned.
When told that polling cards were sent to magazine subscibers, names from the telephone directory and new car owners they soon recognise that the poll may have only questioned those who were already disposed to vote for Landon
The whole saga of the 1936 ‘Literary Digest’ poll is a great example of huge samples not always being better, and that if your sample is biased in some way to start with mking the sample bigger is just going to make the result even more inaccurate. An interesting postscript to this story is that one poll from 1936, over only 1000 people, was accurate. It was run by a man called George Gallup, whose organisation is still working today. The Literary Digest folded soon after the 1936 election !
Teaching Rational Thinking 
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