Understanding Possible Change and EPDs

by Ryan Boldt, Director of Breed Improvement A big part of the discussion about EPD possible change has to do with an animal’s accuracy value that corresponds to its EPD.

Accuracy is defined as the relationship between the animal’s estimated progeny difference and its “true” progeny difference. Accuracy estimates for each EPD range from 0 to 1 but are displayed as 0 to 100 on the RAAA website. The accuracy value that is calculated for each animal is based on the available information contributing to the EPD calculation.

Possible sources of information about an animal include records on the individual, relatives, progeny and genomic information. As these sources of information become available for a specific animal, the accuracy value for that animal increases. Another possible way to look at an accuracy value is to view it as the reliability or risk of the EPD changing with new information. If an animal has a low accuracy EPD (closer to 0) then the addition of more information about that animal is likely to cause a larger change in that animal’s prediction than an animal with a high accuracy estimate (closer to 100).

The idea of the possible change table is that it provides a range where it’s expected that the true value of the prediction will lie within the published possible change two-thirds of the time. There is a possibility that the animal’s “true” EPD value could fall outside of the range calculated using the possible change table. However, it is important to keep in mind that we expect many animals to move in a favorable direction compared to an unfavorable one. So, there is an equal probability that an animal’s progeny will outperform his EPD that is estimated before any progeny are produced. Take for example the two bulls in Table 1. Each bull has a weaning weight EPD of 25, Bull A’s accuracy level is 20, and Bull B’s accuracy level is 80. Based on the possible change table on the RAAA website, we would expect that the true EPD value of Bull A would lie within the range of 12 to 38 pounds (25 +/- 13) and Bull B’s true value would range from 21.7 to 28.3 (25 +/- 3.3).

Using this table is simple to do and, based on the current information, helps to see where we would expect the value of an animal to fall if infinite information was available.

Another important thing to be drawn from this example is that although these two bulls have identical EPDs currently, there is a chance that with more information, Bull A’s EPDs could get estimated higher and out of the range of Bull B’s estimate as more information became available on Bull A.

When using EPDs, it is important to keep in mind that the EPD estimates represent the average difference between two animals if they are mated to genetically similar individuals. Therefore, possible change represents the range that we would expect this difference to fall into.

This does not correspond to expecting lower accuracy individuals to have more variability in phenotypic values in their calf crop compared to high accuracy animals. It simply means that with higher accuracy animals, we have more information and have a more reliable assessment that the estimated difference is closer to the true difference for this animal.

In summary, EPD possible change values give an estimate based on the current information where we would expect the “true” progeny difference of an animal to lie two-thirds of the time. As more information is available on an animal, we expect his accuracy value to increase and approach closer to the true EPD value for that animal. We also expect that just as many animals’ EPDs will improve as more information becomes available on them as those who do not move in a favorable direction.

Additionally, possible change of an EPD does not correspond to variability in phenotypic observations in a calf crop. We would expect to have the same phenotypic variability regardless of EPD accuracy value. When we take this information into consideration, it allows for more effective use of EPDs, which are the best tool available to make selection decisions. //

Table 1. Example of Possible Change for RAAA Bulls WW WW EPD Possible Change Possible Change

Bull EPD Accuracy Value Range

A 25 20 +/-13 12 to 38

B 25 80 +/-3.3 21.7 to 28.3

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