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IWG Bulletin Ref: 2000-7

Understanding the Normal Distribution, Mean, Standard Deviation and Coefficient of Variation

Introduction
An understanding of some basic principles of statistics is important for understanding and interpreting Objective Wool Measurements from individual sheep and for IWTO test certificates.
When comparing one sheep to another for mean fibre diameter (MFD (microns), standard deviation, co-efficient of variation, or for any other wool measurement, it is important that the sheep have run under the same environmental conditions since the last shearing (ie same paddock) and they are from the same age group.

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IWG Bulletin Ref: 2000-6

Understanding the terms Accuracy, Precision & Bias

Introduction
The terms Accuracy, Precision and Bias are important in understanding and interpreting objective wool measurement (both in laboratory and in field conditions) and when comparing one result to another.
Small differences exist between instruments and operators even under ideal conditions for equipment, procedures and quality control.

• Accuracy is defined as how close you are on average to the reference value or "true" result.
• Precision is defined as the repeatability of a result and relates to how variable the results are compared to the average. This is often expressed as a 95% confidence limit or variance.
• Bias occurs when there is a consistent difference between the measured value and the "true" or reference value.

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IWG BULLETIN Ref: 2000-4

Accuracy of the OFDA2000

Definition of Accuracy: How close you are on average to the reference or "true" result.

• Accuracy can be defined as how close you are on average to the reference or "true" result. In practice we must use IWTO core testing of a complete lot as the "true" result. However, because all individual results have a degree of random error associated with them, we have to look at comparing averages over several lots to get a true estimate of accuracy.
• On average, we expect on-farm estimates for lots to agree with the core tests within ± 0.2 µm, but individual comparisons will vary by as much as ± 0.8 µm. Similar performance is achieved by BOTH current on-farm systems. *
• Normally the agreement is better for the lots in the middle of the diameter range on any property. The finer lots will almost always have coarser results when core-tested, irrespective of which fleece testing method is used.

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IWG BULLETIN Ref: 2000-3

Precision of the OFDA2000

Definition of Precision: The repeatability of a result.

• The published micron precision estimate of an 18 micron 5-bale line using OFDA2000 is approximately +/- 0.35 microns.
• The published precision for an IWTO core test is similar.
• Therefore under perfect conditions, when the 5-bale OFDA2000 test average is compared to core test results, you can automatically expect variation of up to +/- 0.5 micron (a statistical fact of life).

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IWG BULLETIN Ref: 2000-5

Measurement Matters

The two most-heard comments in relation to wool testing are:

1. "The test I did on that sheep last week was so and so and now it's different".
2. "I sent half the sample to one lab the other half to another lab and got two different results."

Those comments are addressed with the following information:

1. "The test I did on that sheep last week was so and so and now it's different"

The components of fibre diameter variation can be discussed under 4 main headings.*

• Differences between animals.
• Differences between positions of the sheep.
• Along the fibre.
• Between fibres.

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