Average Earnings Index (AEI). The average earnings per starter in relation to the average earnings of all starters by all sires in the same years. It's adjusted for inflation and is calculated so that 1.00 is the average for sires. Thus an AEI of 2.00 is twice as good as the average. Since a sire's average earnings per starter is derived by dividing total earnings by the number of starters, the index can be distorted by the presence of one outstanding runner.
     Sire Production Index (SPI). It's based on average earnings per start rather than average earnings per starter, but like the AEI, it compares the runners' average with that of all other starters in each year (1.00). It is adjusted for inflation, and it can't be distorted by a single big winner because the sire's runners are given individual rating that are added and then divided by the number of starters. It has its own distortion: a runner whose career ends before age and infirmities cause the inevitable decline in class gains a higher rating than a sound one with a long career. Thus it distorts in favor of unsoundness, though that weakness is partially offset because only runners with at least three starts are considered. It is an index developed and offered by Bloodstock Research Information Systems (BRIS). 
     Comparable Index (CI)-A companion to the AEI. Since a sire's performance is influenced by the quality of the mares to whom he's bred, it would be useful to be able to compensate for that variable. The CI is the AEI of the runners produced by a sire's mates when they were bred to other sires. The pairing of the AEI and the CI attempts to stabilize that important variable, with a sire credited with "moving his mares up" if his AEI is higher than his CI--and vice versa. But the CI doesn't take into account the quality of those other mates. A stallion bred to mares whose other mates were world-class sires would have a hard time matching their performance even if he produced superior runners. For example, even if such a sire had a 3.00 AEI (three times the average), his accomplishment would be denigrated if his CI were higher still. The CI is useful, but not as useful as it appears to be.
     Average earnings per starter-This is one of the crudest of the popular measurements. It's calculated by adding all the purse money earned by a sire's offspring and dividing that by the number of his runners. It's hard to know what this average means, because a handful of big winners (or even just one) can make the sire look better than he deserves. This statistic should always be paired with . . .
     Median earnings per starter. This number is a kind of mirror image of the one above; it corrects for its distortion by introducing one of its own-it gives almost no credit to a big earner. It is the earnings total at which half a sire's runners have earned more and half have earned less.
     Superior runners. This figure identifies runners with average earnings per start in top three percent of all runners who started at least three times. It has the same biases as the SPI, but it is useful. It can be found in BRIS products.