Inferring and Explaining

99 As the 1936 election approached, they once again conducted a massive poll. Take a look at the rel- evant data. e 1 . The Literary Digest mailed out more than ten million straw vote ballots. e 2 . Their sample was drawn primarily from automobile registration lists and phone books. 4 e 3 . “Over 2.3 million ballots were returned.” 5 e 4 . 55 percent planned to vote for Alf Landon, 41 percent for Roosevelt, and 4 percent for Lemke. Tis led to their conclusion that voters over- whelmingly favored Landon and their cover story prediction that he would win the election. Tey made a classic inference from a sample to a population. e 1 . Literary Digest sample strongly favors Landon. t 0 . Voters, nationally, strongly favor Landon. Bad luck for the LiteraryDigest ! You, of course, know that Alf Landon never became president. I’ll bet a good number of you have never even heard of him before. Roosevelt crushed Landon in the general election 61 percent to 37 percent. What went wrong? Te Digest ’s sample was horribly biased. Not because they were prejudiced or had some ax to grind but because the way they selected the names and addresses was far fromrandom—not the technical randomness that we almost never fnd, but the practical randomness that good polling requires. Te clue is in e 2 . Tis was, afer all, the height of the Great Depression. Poor people were much less likely to own a car. And even phones were then considered not necessi- ties but, in a sense, luxuries. Again, poor people were much less likely to have phones. What the Literary Digest had unintentionally done is mea- sure the sentiments of relativelywealthy voters, not voters in general.Tis suggests the following rival explanation: statIstICs t 1 . Wealthy voters strongly favor[ed] Landon. It is well known in political science that wealth- ier voters tend to vote for Republicans and less wealthy voters for Democrats. It’s hardly sur- prising, therefore, that a sample of voters biased toward the Republican Party tended to favor the Republican candidate. Tere was a second source of bias in the sample that is less well discussed in academic circles. Te whole poll depended on what stat- isticians call the “response rate.” Te Liter- ary Digest sent out a truly amazing number of straw ballots—more than ten million. Tey got a pretty good response too—almost a quarter. But we should ask ourselves if there was any- thing special about those 2.3 million who took the trouble to mail their ballots back. It seems reasonable to suppose that they were more edu- cated and politically concerned. So we have a second rival explanation: t 2 . Better educated and politically concerned voters favored Landon. And, indeed, t 1 and t 2 nicely complement one another and suggest a more comprehensive rival: