Some people are excited (positively or negatively) about Nate Silver’s column today giving a probability of a GOP takeover at 60%. To cut to the chase: I do not think that number means what you think it does.
Here are three big things to think about.
1) Five coin tosses. At this point, Senate control comes down to as few as five* races: AK, CO, IA, KY, and LA. Think of these races as coin tosses. Then Democrats have to win 3 out of these 5 tosses to retain control. (I’m simplifying matters, but not by much.) These coins are not perfectly fair, and the overall situation is a little unfavorable to Democrats. That is basically the amount of uncertainty expressed in Silver’s probability.
Fundamentally, any probability in the 40-60% range is a numerical way of saying “I don’t know.” (Just to poke at the scar a bit, “I don’t know” is what Silver should have said when he intimated that Brazil would probably beat Germany in the World Cup. We all know how that turned out.)
2) The certainty fallacy. Silver has done something common among paid writers, which is to do what it takes to attract eyeballs. He has rounded a probability that is barely over 50% to make the statement that one side is ahead.
This problem is endemic to journalism. Nuance with probabilities is just lost on most people. But writers are just trying to appeal to their readers. The real headline is “WE DON’T KNOW” but somehow that is not what gets the lead. Really, he’s caved in to all of you!
Basically, whenever you see a probability like that, you should mentally say “plus or minus 20%” just to get the right idea. Of course, if the probability gets above 80% that doesn’t make sense any more. At that point, it’s OK to say that one side is ahead.
3) One of those coins might say “D” on both sides. As I’ve pointed out before, the biggest problem is that we don’t know how Alaska will turn out. The only recent Alaska poll shows Sen. Mark Begich (D) bounding out to a 12% lead over likely nominee Dan Sullivan (R), probably because of a gaffe on Sullivan’s part.
If Begich really is ahead, that translates to Democrats only having to win 2 out of 4. That gives a pretty accurate intuition for where the Senate race actually is as of today.
Make no mistake: at the moment, the battle for Senate control is super-close. If national opinion swings as little as 2 points in either direction, uncertainty about Senate control will evaporate. As I’ve written before, even if that happens, don’t be surprised if both sides end up with 49 to 51 seats each in 2015.
*Just for purposes of building up a clearer intuition about probability, I’ve left out AR, GA, and NC. Of course, here at PEC our Meta-Analysis considers all races at once. But the bottom-line answer is exactly what I’ve said. So for purposes of discussion today, I’ll leave those races out.