What is the variant score (the number displayed next to the nick rating) and how much weight should I give it?
The TrueNicks variant is a function of the Sire Improvement Index (SII) and Broodmare Sire Improvement Index (BSII).
The first step is to establish an SII, which is done by comparing the percentage of stakes winners to starters sired by a sire/sire line out of all mares by a broodmare sire/sire line, with the percentage of stakes winners sired by the same sire (or same representatives of the sire line) out of all other mares. An SII of 2.0 indicates the sire (or sire line) has twice the percentage of stakes winners out of mares by the broodmare sire/sire line as he does when bred to all other mares.
Next, the BSII is established by comparing the percentage of stakes winners to starters sired by a sire/sire line out of all mares by a broodmare sire/sire line, with the percentage of stakes winners produced by those mares when bred to all other stallions. A BSII of 3.0 indicates that that the percentage of stakes winners out of mares by the broodmare sire/sire line when bred to the chosen sire/sire line is 3.0 times as high as the percentage of stakes winners produced by that group of mares when bred to all other stallions.
The TrueNicks rating is calculated by multiplying the SII by the BSII. In the example above, an SII of 2.0 and a BSII of 3.0 would result in a TrueNicks score of 6.0, which translates to an A++ TrueNicks rating.
It is important to understand that the rating scale is not linear, so while a below-opportunity rating will have a variant of 0.01 to 0.99, an above-opportunity rating can have a score from 1.01 up to numbers as high as 500.00 and beyond. Generally, extremely high scores are the result of a mating which has had considerable success with limited opportunity. The best guide to the potential success of the nick is the letter ranking which has been evolved through careful study of the relationship between the general population and the stakes winning population. Note that only 13% of the entire Thoroughbred population earn “A” rankings (A to A++) while 37% of stakes winners rate as “A’s.” Horses rated “B” or better (B to A++) represent just 30% of the entire population, yet three out of four (77%) stakes winners rank “B” or better. Almost half of Thoroughbreds in general–44%–are on the low end of the scale (rated “C” through “F”), yet only two in 25 stakes winners (8%) have these lower rankings.
Does class (or any other factor) impact the TrueNicks rating?
The TrueNicks rating is purely a reflection of the degree of affinity between the sire/sire line and broodmare sire/sire line. Thus, it is best used to differentiate between matings or individuals within a similar commercial range. This reflects the considerations that breeders and buyers are likely to face in “real world” situations. For example, it is more likely that a breeder considering using Giant's Causeway, might also consider Distorted Humor and A.P. Indy – horses at a similar commercial level – rather than alternative stallions in the $20,000 range, and vice versa.
When planning or evaluating matings, other factors such as a balance of aptitudes (speed, stamina, preference for dirt, turf, or all-weather tracks), inbreeding, line breeding and outcross, and conformation considerations must be kept in mind. TrueNicks is a tool to be used by the Thoroughbred breeder and owner and while studies of over 100,000 horse demonstrate that there is a high correlation between the affinity between the sire/sire line and broodmare sire/sire line that is information that cannot be ignored, it must be given the appropriate weight when considering all other factors that influence eventual racetrack performance.