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Using Accuracy Values in Ram Selection

The Guide to Accuracy Values

An EBV predicts the breeding merit of an animal for a specific trait. The degree to which this EBV reflects the “true” breeding merit of the animal depends on how much we know about its performance relative to the rest of the population.

Why accuracy matters
Accuracy Values indicate the likelihood of an EBV changing (up or down) as more information about the animal becomes available. For any trait, the accuracy of the EBV is influenced by several factors:-

  • Amount of information for the animal
  • Amount of information from relatives
  • Heritability of the trait
  • Amount of information from traits correlated with the trait of interest and the strength of these correlations
  • Number of animals being compared (contemporaries)

Animals with high accuracy values
Selecting breeding stock with high accuracy values minimises risk in breeding decisions. Widely used stock sires, e.g. reference rams with lots of recorded relatives will have high accuracy values. Fully recorded ram lambs with lots of recorded relatives will have acceptable accuracy values.

Animals with low accuracy values
An important feature of Signet’s breeding evaluations is that they are risk averse. EBVs based on limited amounts of information get adjusted back towards an average figure until more data becomes available. Among those animals with low accuracy values there may be individuals with good genetics, but a lack of performance data means they are difficult to identify using EBVs.

Accuracy, connectedness and comparisons between flocks
Accuracy is not a direct measure of the quality of connections between animals in the recorded population. This is better achieved using “flock connectedness”. However, the value of across-flock comparisons between animals with low accuracy will be of limited benefit.

Presentation of accuracy values
Accuracy values are presented for each EBV and expressed as percentage points ranging from 0 to 99. In the example below are two rams with different EBVs and Accuracy Values. The ram lamb has superior EBVs, but his lower accuracy values indicate there is a higher chance that they may change (increase or decrease) if he has progeny recorded in future evaluations.

Recorded Stock Ram – with 80 progeny
EBV Accuracy
Scan weight EBV 4.6 92
Muscle Depth EBV 2.8 86
Fat Depth EBV 0.2 87
Index £1.98 88


Recorded Ram Lamb
EBV Accuracy
Scan weight EBV 5.0 78
Muscle Depth EBV 3.2 67
Fat Depth EBV 0.3 69
Index £2.15 75


Accuracy values indicate how similar an animal’s EBVs are to its true breeding value.

Breeders can use accuracy values to predict the likelihood that an animal’s EBVs will change over time

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