The Comparable Index (CI) measures the average earnings of progeny produced from the annual book of mares bred to a sire, when those same mares were bred to other sires. In other words, CI indicates the producing quality of the mares bred to a stallion.
To understand CI, you must first be familiar with the Average Earnings Index (AEI). The CI for a stallion can be thought of as a composite AEI for the progeny of all of the mares in his stud book for a given year. If those mares' previous foals by other sires have collectively won three times the average annual earnings of all Thoroughbreds in their individual classes, the sire's CI for that year's book would be 3.0. And if the mares in his book are poor producers -- say, they earn half of the average in their sex/age groups -- the stallion's book would be a 0.5 CI.
Reviewing the CI, breeders will often note that a particular stallion "improves" his mares, while another stallion might breed mares that improve him. What these breeders are doing is making a simple comparision of the stallion's AEI and CI figures. The AEI-to-CI ratio indicates how well a stallion's progeny have performed on the racetrack (straight earnings) compared to how well those foals' half-siblings fared in the same time period. A high AEI-to-CI number means that the stallion's colts and fillies are outperforming their dams' other foals and is considered proof that the stallion improved his book of mares.
Because of CI, the September issue of The Blood-Horse MarketWatch is one of the more popular editions of this professional newsletter. In that month, MarketWatch presents a Quality of Dams feature that analyzes stallions by the strength of the book they've attracted. (I won't confuse the issue here, but the feature looks at other factors -- including Class Performance Index, or CPI -- as well as CI.) While stallion farms are interested to see where their studs land in the rankings, the charts are most anticipated by broodmare owners.
In my mind, last year's stallion CI standout was Perfect Soul (IRE) (SRO). Despite standing for only $15,000, his 3.06 CI for foals born in 2008 beat every stallion up to a $100,000 stud fee except for one (his fellow Northern Dancer-line stallion Dixieland Band). If I were a commercial breeder or even a pinhooker, I would take notice. Here's why: it's fair to assume that the Perfect Soul foals of 2008 will be standout runners in 2011. (While he's unlikely to throw extreme precocity and therefore might not have top juveniles in 2010, his own talent and the fact that his mares are proven producers mean that we should expect a solid crop of talented sophomores a year later.) I'd be willing to gamble that some big wins in early 2011 would increase demand for Perfect Soul 2-year-olds in the spring and for yearlings in the summer and fall. So a mare owner in the recent breeding season might have been smart to consider sending a commercially-bred mare to Perfect Soul for a 2010 foal (i.e., a 2011 yearling). Similarly, a pinhooker in 2010 might keep that in mind and give special attention to 2010 yearling bargains that would be resold as racing prospects during 2011.