Example: Selection Bias in Politics

Lead Author(s): Jeff Martin, MD

Examples of Selection Bias in Political Studies


1948 Presidential Election

The 1948 preelection surveys used various methods to find subjects and most of these surveys found that the largest % of persons favored Dewey. Although this was in the field of social science, the errors made in the pre-election polls really turned the entire science of sampling on its head and ushered in an era where we live today where we recognize the importance of representative (or random) sampling.

California Recall Election of Gov. Davis

(2) California Recall Election of Gov. Davis: As shown below in the 2003 California governor recall election, about a week before the election the San Francisco Chronicle reported that in a sample of 894 persons 57% favored recalling the governor.


A week later the election was held and now we can tell how good our sample was:

Why Are Election Polls Important?

Why are these election polls such a great way to learn about sampling? Because they are one of the few situations that we actually get the truth about the source population, on election day, and can thus get a handle on whether there was bias in the survey. Unfortunately, we don't get this luxury in clinical research.