EXAMPLE 3 The effect of race

In 1989, New York City elected its first black mayor and the state of Virginia elected its first black governor. In both cases, samples of voters interviewed as they left their polling places predicted larger margins of victory than the official vote counts. The polling organizations were certain that some voters lied when interviewed because they felt uncomfortable admitting that they had voted against the black candidate. This phenomenon is known as “social desirability bias” and “the Bradley effect,” after Tom Bradley, the former black mayor of Los Angeles who lost the 1982 California gubernatorial election despite leading in final-day preelection polls.

This effect attracted media attention during the 2008 presidential election. A few weeks before the election, polls predicted a victory, possibly a big one, for Barack Obama. Even so, Democrats worried that these polls might be overly optimistic because of the Bradley effect. In this case their fears were unfounded, but some political scientists claimed to detect the Bradley effect in polls predicting outcomes in primary races between Barack Obama and Hilary Clinton (for example, in the New Hampshire primary, polls predicted an 8 percentage point Obama victory, but Clinton won by 3 percentage points).