What is Relative Percent Difference (RPD) used for?

• A. A comparison of two duplicate results to determine how precise they are; expressed as a percentage.
• B. A measure of how much a sample's concentration deviates from the target value.
• C. A method to verify the accuracy of a calibration curve.
• D. A statistic indicating the variability between different laboratories.

Answer: (A)

Relative Percent Difference measures how consistent two repeat measurements of the same sample are. It expresses the closeness of duplicates as a percentage, reflecting precision or repeatability.

The idea is simple: take the absolute difference between the two results, divide by their average, and multiply by 100. For example, two measurements of 102 and 98 differ by 4, and their average is 100, so the RPD is 4%. A small RPD means the duplicates are very close, indicating good precision; a large RPD means the measurements vary more, indicating poorer precision.

This is about precision within a single method and lab. It does not tell you how far a measurement is from a true target value (that’s accuracy), it isn’t a check of calibration curve accuracy, and it isn’t a measure of variability between different laboratories.