As my regular readers (all four of you) know, one of the research projects I'd like to tackle is an evaluation of the existing measures of breeding success, and ultimately coming up with some statistics that do a better job of predicting future success. As I mentioned yesterday, the main challenge has been finding the data I need to study it. This morning I had a very minor breakthrough. Although BRIS is the data provider for The Bloodhorse, they don't always present the data in the same format. It turns out that BRIS has a version of the leading freshman, leading second crop, and leading third crop sires where they include SPI. As far as I can tell, SPI appears to be the same thing as AEI, although I haven't yet checked the numbers to see that the calculations are being done exactly the same.
I took list of leading first crop sires of 2007 and the list of leading second crop sires of 2008, both ranked by SPI. Of the 75 names on each list, there were 59 in common. The correlation between the SPIs was .65. Removing the horses with very few starts (less than 20 starts in the 2007 list and less than 30 starts in the 2008 list) didn't make much difference. The remaining 34 horses had a correlation for their SPIs of .63. On the surface, these correlations look pretty high, and indicate consistent performance by sires across crops. Of course, the problem is that a lot of that consistency may be coming from the fact that some sires are covering better mares than others, so we'll need to make an adjustment for that before we can have any confidence in the results we're seeing.