How is Distribution Uniformity (DU) calculated in catch-can test?

• A. DU = (average of lowest quartile) / (average of all cans)
• B. DU = (average of highest quartile) / (average of all cans)
• C. DU = (sum of all cans) / number of cans
• D. DU = (average of middle half) / (average of all cans)

Answer: (A)

In a catch-can test, Distribution Uniformity measures how evenly water is applied by comparing the poorest portion of the system to the whole. It is calculated by taking the average discharge of the lowest 25% of cans and dividing it by the average discharge of all cans, usually expressed as a percentage. This focuses on the portion receiving the least water and shows how close that value is to the overall mean, which is what DU is meant to reflect. For example, if the overall average is 1.0 L per can and the lowest quartile averages 0.8 L, DU is 0.8/1.0 = 0.8, or 80%. The other options don’t measure uniformity the same way: they either use the wrong quartile, or simply compute an overall mean that doesn’t account for distribution at the low end.