Sunday, May 20, 2007

The Preakness: Odds and History

I was struck by the headline in The New York Times on the day of the Preakness Stakes: referring to Kentucky Derby winner Street Sense, it read, "Favored in Preakness by Odds, Less So by History."

The article, by Joe Drape, shows the opposite.

It opens with a reference to Street Sense's status as the 7-to-5 favorite in the Preakness. That means that the odds gave Street Sense just under a 42% chance of winning. Like other sports betting markets, that of horse races has a fantastic track record, so we can reasonably expect horses with 7-5 odds to win about 42% of the time.

The article then moves to this paragraph, quoting Street Sense trainer Carl Nafzger:

“What I know is that I have a 9-to-1 chance to win,” he said, referring to the number of starters in Saturday’s race. “And that’s a lot better than it was at Churchill Downs for the Derby when we were 19-1.”

Because I am a generous man and have a good dessert in my belly, I will assume that Nafzger likes to toy with reporters rather than that he is as silly as this comment. (By this logic, horse owners might as well toss any old entry into the Preakness; heck, but a bunny in the gate--it's a lot cheaper to feed than a horse, and it will still have the same chance of winning!) Even so, this is an inspired bit of nonsense. It combines obviously false egalitarianism with a classic confusion of probability and odds ("9-to-1 chance" rather than one in nine).

Drape makes more sense in the paragraph that follows:

History suggests that the odds are much better than that for Street Sense: 52 percent of Preakness winners were sent off as the post-time favorite, as Street Sense certainly will be. In the past 10 years, six Derby winners have won the mile-and-three-sixteenths race and headed to Belmont Park with a chance to sweep the Triple Crown.

And here the more interesting point arises: if pre-race favorites had won 52% of Preakness runs, and Derby winners have done even better in recent races, then Street Sense's 7-5 odds reveal a weaker favorite than one would expect, a horse given less of a chance to win by bettors than the performance of similar horses in the past would indicate.

"Favored in Preakness by Odds, Less So by History"? Nope--favored by history, a little less so by the odds.