According to the Boyd & Harris criterion, the CI should not exceed ____ times the width of the RI.

• A. 0.1
• B. 0.2
• C. 0.5
• D. 1.0

Answer: (B)

The idea being tested is how precise the estimated reference interval should be relative to the width of that interval. In this context, Boyd & Harris set a practical threshold to ensure the reference interval is estimated with enough precision. The rule is that the confidence interval around the reference interval estimate should be no wider than about one-fifth of the reference interval’s width. Expressed as a ratio, the CI width to RI width should be ≤ 0.2. This keeps the uncertainty small so that a new animal’s value can be reliably judged as within or outside the normal range.

So the accepted threshold is 0.2. Choices that imply a smaller threshold would be stricter than needed, and those allowing a much wider CI relative to the RI would reduce precision and reliability in classification.