EPD’s …How Accurate is Accurate enough?

EPD's ...How Accurate is Accurate enough?


There’s some confusion about the term “accuracy” when it comes to EPD’s. It would make common sense that if something had a low accuracy that it would not be very reliable…correct? No, that is not how the term ” is used with EPD’s. EPD Accuracy simply describes the Trait Value prediction in greater detail. A very important concept to remember is that EPD’s are correct, or better than predicted, 85% of the time….regardless of the Accuracy. Accuracy defines a range, plus or minus the trait value that can be expected. Just like Las Vegas, EPDs don’t like to be wrong. The Accuracy Range looks more like the point spread used in sports betting. A “low accuracy” represents a wider range of acceptable outcomes. For example, according to the Table 7 of 2013 EPD Producers Report (Table 8 for suris) an accuracy of .3 (low) indicates that the result could be .82 of a micron less than or greater than the trait value predicted. In other words a relatively “Low Accuracy” prediction is just as reliable as a high accuracy prediction. The only difference is the low accuracy prediction is providing a wider range of what can be expected.

So, I think the next logical question is “How reliable are EPD predictions anyway?” As I stated above, my personal, short, unscientific, quick and simple answer is that EPD’s trait values will be as good or better than predicted at least 85% of the time. However, if you were to ask your college statistics professor he would say that EPD predictions are calculated to within 2 standard deviations (68%). The remaining 32% will result in an outcome that is better or worse than the prediction. I frequently refer to this “unknown group of 32%” as the “freaks”. There are good freaks and poor freaks. Being the optimist that I am I say that at least half of the 32% will be good freaks and feel confident with expecting 85% reliability. This is the link to the 2013 Producers Report.

How does fiber diameter affect fleece weight and density?

How does fiber diameter affect fleece weight and density?

It took us a few years to catch on to the degree fiber diameter plays on fleece weight. Early on we had heard rumblings how the fleece show scoring system had become counter-productive to rewarding density but it wasn’t until I did the math did the reality hit home. It became instantly obvious that if we wanted to breed for fineness and density we had to find another way to identify leading producers.

The attached PDF document that helped us visualize the role fiber diameter plays in determining density through three of the most commonly used methods, weight, sight and feel.

The first issue is that fiber diameter is two dimensional. When dealing with diameter, dimensions take on a very different value than what most of us deal with every day. In determining the area of a round object, like fiber, a diameter of 12 is not half or 50% of 24. It is 1/4 or 25%. An area with a diameter of 18 is 1/2 or 50% the diameter of 24. So, in other words if your beautiful,  10lb, 24 micron fleece could be magically changed to 18 micron it would loose 1/2 it’s weight AND VOLUME. Also, If you were to physically examine two animals, one with a MFD of 18 and the other 24,  side by side, without the knowledge of the actual MFD, the 24 micron animal would most likely be chosen as being more dense, when actually they would be the same density. The 24 micron fleece described only has more volume and weight, not more follicles. What this demonstrates is that if  fleece weight is used without a proper relationship to fiber diameter then one would be selecting for increased fiber diameter.

An examination of the three diagrams on the bottom of the page demonstrates how misleading using volume (fills hand when grasped) or amount of visible skin when the fleece is parted can be without exact knowledge of what the fiber diameter is that you are looking at or comparing.

The chart at the top of the page shows the percentage relationships based upon diameters.

Fiber micron comparison_10_09

What Are EPD’s (Expected Progeny Difference)

Expected Progeny Difference = EPD_015EPD’s represent the breeding value of an animal based upon it’s own merit and production along with up to four generations of its pedigree and relatives merit. The EPD trait value will be within the predicted value OR better about 85% of the time. 85% are great odds for making breeding decisions!

Its easy to get EPD values mixed up or confused with histogram data. Histogram data represent visible traits of an animal at a particular point in time. EPD trait values represent how offspring will perform as compared to the entire group in the program. Regardless of age or sex. Program size is currently 27,000 and growing.