In my last post I talked about one approach to determining which sires tend to produce sound offspring, and some potential pitfalls to studying the the topic using that approach. Here I'll discuss some steps that can be taken to ensure that a study that uses this approach eliminates the first (and most complex) of the biases introduced by those issues:
Offspring of better sires will tend to retire earlier due to breeding value.
One thing to keep in mind is that this will affect males and females differently. The effect will be strongest among males, but only those with enough ability (or good enough breeding) to be sent to the breeding shed. In females, the effect will be weaker, but will affect virtually all of them. So we can't fully solve the problem by stuying just one sex or the other, but perhaps by limiting the study to males who are not ultimately used for stud duty. Another option would be to look at geldings only...but then we run into sample size issues...particularly for the top sires, where owners will be very reluctant to geld their sons.
One possible approach to eliminating this as a factor would be to look at average starts per year of racing, instead of average starts per career. If we did that, we'd need to make sure to control for age in some way though, since horses will tend to race more in the prime of their career than as two year olds. One bias in the data we should be aware of here is that cheaper horses will tend to race more, regardless of soundness. So the pampered children of star sires like AP Indy may come out looking less sound than they really are.
Next post, I'll start looking at the other three issues I brought up regarding this research topic.