Using genetic ratings: the Confidence interval

Confidence interval, example of a stallion (2018 ratings: SJ +23 and CD 0.77)

Genetic indexes are rarely 100% accurate (value of CD* = 1). Even if the index is still the most likely measure of a horse’s genetic worth, there is still a probability that the real genetic worth could be higher or lower than the value of the rating. The confidence interval is the interval within which the horse’s true genetic worth is likely to be.

The higher the CD is, the smaller the confidence interval, and the more the genetic index of the horse can be considered as an assessment of its hereditary value.

Confidence interval at 95%= Genetic rating+/-1,96 sigma A √(1-CD)
Sigma A being the standard deviation of the population for the characteristic considered.

(CD = *Determining Factor)

By combining the genetic rating and the related CD (determing factor), from the confidence interval as presented above, the lower limit can be calculated. This lower limit represents the threshold value above which there is a 97.5% chance of finding the stallion’s genuine worth. The lower limit is the basis for genetic ranking of breeding stallions (see table below). This ranking has been established in the disciplines of Show-jumping, Eventing, and Dressage.

Thus Genetic Ranking guarantees not using a stallion with a too low a value. The figure below illustrates the case of an identical show-jumping rating (BSO +25) for two stallions presenting different specifications, a young stallion with a CD of 0.56, and a confirmed stallion with a CD of 0.98.

Genetic ranking (2018) of two stallions with the same genetic rating, but  different CD (Determining factor)

Genetic classification favors the older breeding males with a same genetic index. However, using older stallions lengthens the generational interval, which in turn penalizes yearly genetic progress. To increase the value of young stallions, another interpretation of the confidence interval can be used. Instead of minimizing the chances of poor genetic value (the purpose of genetic classification), value betting maximizes the chance of having an outstanding stallion. Value betting assesses the probability of placing the genetic worth of a stallion within the top 2% of its generation per breed, with regard to the genetic index and CD (determining factor). The top 2% threshold is of:

•  + 21 for Selle Français horses
•  +10 for Anglo-Arab horses

With an equivalent genetic rating, young stallions with a low CD are promoted by value betting. Value betting is founded on hope. It measures the chances of the stallion being among the elite of its breed. The figure below illustrates the case of two “very good SJ” stallions, with a very different value bet. The first, UNESCO DU FEU is a young stallion with a 73% chance of having a genuine genetic value of over +21, whereas the second stallion, DIAMS DU GRASSET has no chance. Both are certainly very good for show-jumping, as neither are likely to have a genuine value below 7.5.

Value bet for two “very good”  show-jumping stallions (2015 ratings)

For some stallions who produce both Selle-Français (SF) and Anglo-Arab (AA) offspring, there can be two value betting values:  SF Value bet, and AA value bet.
In this case the stallion is Anglo-Arab.