Standardised Values (Indexing)
For beef cattle the base year to which all EBVs are compared is fixed at 1980, but the current EBV averages are constantly changing and this can sometimes make it difficult for breeders to explain EBVs to potential customers.
Consequently, some of the major breed societies asked Signet to present the EBVs in index form based around an average of 100 with an approximate range of 70 to 130.
While this is a beef example, exactly the same principal can be used for sheep - with standardised values generated accordingly.
Table 1: Limousin EBVs showing a bull with above and below average EBVs
These “EBV Indexes”, along with EBVs in their measurement units, are published in sale catalogues and on pen cards where requested by the Breed Society.
To calculate EBV Indexes the following formula is used:
For the three traits where a low EBV is better (birth weight, gestation length and fat depth) the standard deviation is multiplied by –1.