Nate Silver contemplates the differences between live operator and automated polls:

The original Bradley Effect, named for former Los Angeles mayor Tom Bradley, occurs when respondents in surveys are asked about socially desirable behaviors, such as being free from racial prejudice. Although the racial version of Bradley effect itself is probably a thing of the past, social desirability bias may manifest itself in other ways. Automated polls have sometimes shown relatively lower levels support for gay marriage initiatives, for instance, in states like Maine and California. Homophobia is fairly common, but has become socially undesirable; the purveyors of the automated polls have sometimes claimed that their respondents are free to be more honest when there's not another human being on the line. If the theory holds, automated polls might also provide a setting for voters to be more honest about their feelings on marijuana use, another behavior that is probably more widespread (and privately tolerated) than it is socially acceptable. If so, that would be good news for Prop 19.

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