November 2, 2012

What a 72% chance means

For those of you who have been living in a cave on Mars with your eyes closed and your fingers in your ears, the US is having a presidential election.

Nate Silver, a former baseball nerd who has climbed to fame by computing averages of opinion polls, said earlier this week that  Obama had a 72% chance of winning.  Old-fashioned political pundits disagreed, saying the race was too close to call (they also accuse Silver of just computing averages of opinion polls).

Andrew Gelman, a stats professor at Columbia University, has an opinion piece in the New York Times explaining what 72% means:

What I’m saying is that I can simultaneously (a) accept that Obama has a 72 percent chance of winning and (b) say the election is too close to call. What if the weatherman told you there was a 30 percent chance of rain — would you be shocked if it rained that day? No.

For Kiwi readers, a useful analogy might be rugby.  The All Blacks have beaten Australia in 70% of their games overall, and 75% in NZ.

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Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »

Comments

  • avatar

    It’s up to close to 81% now, which is fairly strong but still not completely overwhelming.

    Not sure what would be wrong about averaging opinion polls. Makes more sense than taking them in isolation, or ignoring them completely.

    12 years ago

    • avatar
      Thomas Lumley

      The point is that some people thought Nate Silver was claiming some special secret sauce for his predictions, when in fact he’s always been very clear about his methods and they are pretty boring (but sensible and effective).

      12 years ago

  • avatar
    John

    Since I don’t follow sport, unfortunately your rugby analogy didn’t help me understand it any better.

    12 years ago

    • avatar
      Thomas Lumley

      Oh well. It would probably help 72% of New Zealanders, though.

      12 years ago