Can You Explain the Significance of the TN Variant?
Thanks for the email and we appreciate ordering the broodmare analysis report. I'm glad you have found it of utility in your breeding decisions.
To answer your question the TrueNicks Variant is firstly calculated by factoring two separate computations:
Sire Improvement Index (SII), a comparison of the percentage of stakes winners that a sire (or sire line) has achieved with mares by the specific broodmare sire (or sire line), with the percentage of stakes winners that specific sire (or those specific members of the sire line) has achieved with all other mares.
Broodmare Sire Improvement Index (BSII), a comparison of the percentage of stakes winners produced by daughters of a particular sire (or broodmare sire line) with the percentage of stakes winners from the same mares when bred to all other sires.
The resultant figure shows the stakes winner-to-starter production rate of the nick, compared to the stakes winner-to-starter production rate of the sire/sire line and broodmare sire/sire line when bred to representatives of all other lines. Thus, a TrueNicks score of 2.0 indicates that the cross has produced stakes winners at twice the rate that the sire/sire line and broodmare sire/sire line have when bred to all other lines. These ratings are subsequently translated into bands from A++ to F.
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.
From practical experience, in using the TrueNicks Broodmare Analysis Report and the TrueNicks ratings for commercial matings for some time now, we have found that B+ or better ratings are a good starting point for consideration. While comparing variant scores looks interesting, it is really not the most productive way to judge one potential mating over another and if you understand how they are calculated as discussed above, there is not a large difference between horses that have a variant of 355.00 and those with a variant of 3.55. This is why we tend to reiterate the point that using the letter grade is more productive than the variant and it is particularly relevant when looking at a number of high rating options for a given mare.
Faced with multiple options for a mare, what we tend to do is look at the quality of the horses that have been bred on the cross (as they appear in the top 5 horses bred on the cross on the TrueNicks page), and use that as a guide to rank one mating over another. Say for example you have the choice between a B+ mating with the top 5 being some really solid racehorses including some G1 winners and an A+ mating but the top 5 horses being stakes winners of lesser performance. All other things being equal we would probably put the B+ mating ahead of the A+ given that the quality of the horses that the nick has produced is better in the former case. A little bit of intelligent interpretation of the information given like this will help you whittle down the suggestions found on the TrueNicks Broodmare Analysis Report to a manageable number.
Of course from there this must be weighed up with other factors in the pedigree including requirements of speed or stamina, temperament, class and other pedigree factors along with physical considerations of the sire and the dam. The latter is of particular importance as paper doesn't run very fast.
Thanks again for the question and best of luck this breeding season.