What is a selection index?

A selection index is a method to improve several traits simultaneously. As a breeding manager of a large breeding program or a breeding cooperative comprised of many smaller breeders working together, a decision first must be made which traits will be prioritized to be in the selection index. Each trait is assigned a relative level of importance for each of the traits in the selection index. The level of importance is termed the weighting. An adjusted weight is calculated for each trait and the total of each adjusted weight is the index value. The dogs with this highest overall index are the dogs that have the highest likelihood of producing the desirable trait for the collection of traits in the index. The index is provided as a percentile ranking where the higher the percentile ranking is most desirable (100% most ideal, 1% least ideal).

The image below shows a hypothetical index where the allergy EBV is given 2x more weighting than hip quality and elbow quality.   The EBVs are calculated for three traits allergy score, hip quality and elbow quality.  The allergy score is given a weighting of 2 while hip and elbow quality each have a weighting of 1.  This means 50% of the index is on the allergy score and 25% each on hip and elbow quality.  If the principles of selection are followed over multiple generations, it can be expected that the genetic quality for allergy score will increase faster than hip or elbow quality but all will improve simultaneously.