Friday, November 7, 2008

More on Trainer Winning Percentage

In my previous post I mentioned that trainer winning percentages appear to be very stable from year to year. For the 73 trainers who were among the top 100 in the US in wins in both 2005 and 2006, there was a correlation of .83, which means that the list is exceptionally stable. The highest percentage trainers are pretty much the same from one year to the next to an extreme degree. In fact, it was so extreme that I could see the consistency just eyeballing the numbers. Interestingly, the correlation for 'in the money percentage' was slightly lower at .79 for reasons which aren't totally clear to me. I had assumed that it would be a little higher since the sample size of horses in the money is greater.

One thing to keep in mind is that the exact correlation should probably be taken with a grain of salt, since there is a selection bias at work here. The value is probably being artificially increased by the fact that we're not looking at the trainers who had a good enough winning percentage to get them into the top 100 in wins one year, but not the other. This is likely being offset to some degree by the fact that we're also excluding a much greater number of trainers whose winning percentages weren't high enough to make the list in either year. By doing so, we're creating a more homogenous group, which tends to lead to lower correlation values. Either way, if there was no correlation, we'd expect to see a value of 0 in our sample, and a value of .83 clearly indicates a great deal of consistency. It's likely that this is due partly to a trainer's "skill level", but even more to how aggressively they place their horses.

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