Blog Posts by David Rothschild, Yahoo! News

  • Errors, margins and samples: What does the latest poll say?

    The most recent ABC News/Washington Post poll claims a margin of error of plus or minus 4 percentage points; this is a pretty standard margin-range for a serious national poll. In that poll they have Mitt Romney leading Barack Obama by 2 percentage points. What should that mean to you?

    If you interpret the idea of margin of error at face value, it means in this case that there is 95 percent likelihood that the final vote tally would provide Obama with anywhere from 42 to 50 percentage points and Romney with anywhere from 44 to 52 percentage points. Unfortunately, though, that's a gross oversimplification.

    Quoted margin of errors are large and significant. Here is the final difference in the national popular vote for the Democratic and Republican candidates in the last few presidential elections: 7 percentage points in 2008; 2.5 percentage points in 2004; 0.5 percentage points in 2000; 8.5 percentage points in 1996; and 5.5 percentage points in 1992. Some of those elections were close, others not as close--yet the largest difference was 8.5 percentage points. That puts a margin of error of 4 percentage points into perspective; it is meaningful.

    What's more, most people only really care about polling when a race is relatively close. Generally a race is not really close unless it is separated by just a few percentage points--frequently within a 4 percentage point margin of error.

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  • Obama and the generic versus the specific candidate

    Obama (Susan Walsh/AP)Polls are the lifeblood of election-year political speculation--and even though wonkier followers of politics track candidates' progress via shifts in prediction markets that handicap insider knowledge of the election process, opinion surveys also furnish critical insight into how voters approach the choices in a major election cycle.

    Take, for example, the polling work placing President Barack Obama in opposition to a "generic" Republican candidate, versus his showing in opinion surveys against specific GOP presidential hopefuls. Two recent national polls--from Rasmussen and NBC--now show Obama losing by six and four percentage points, respectively, when he's pitted against a "generic Republican" candidate. (Results in Rasmussen traditionally lean Republican, but that's not the case for NBC; the Rasmussen survey has a two-point plus-or-minus margin of error, while NBC's error-margin in plus-or-minus 3.1 percent.)

    Yet the Democratic incumbent is doing much better against most of the

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