Relative Percent Difference for Duplicate Measurements
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Direct answer
Relative percent difference (RPD) is commonly the absolute difference between two replicate results divided by their mean magnitude, expressed as a percentage.
Formula and method
RPD = |x1−x2| / ((|x1|+|x2|)/2) × 100
The calculation keeps full precision through the formula and rounds only the displayed answer.
Step-by-step calculation
- Confirm the two results are comparable replicates.
- Keep units and preparation basis identical.
- Calculate the absolute difference and average magnitude.
- Compare with a method-specific acceptance criterion only if one exists.
Worked examples
Duplicate assays
24.6 and 25.4: gap 0.8, mean 25.0, RPD 3.2%.
Low values
0.10 and 0.15: RPD 40%; small absolute gaps can produce large relative results near zero.
Reporting
State both values, RPD, units and any validated criterion.
Interpretation and edge cases
- RPD alone does not establish accuracy because neither replicate is necessarily a reference.
- It does not replace uncertainty or method validation.
- Acceptance limits are method- and program-specific; do not invent a universal threshold.
Frequently asked questions
Is RPD the same as percent error?
No; RPD compares peers, while percent error uses a reference.
What is a good RPD?
Use the validated criterion for the method and concentration range; there is no universal cutoff.
Why is RPD large near zero?
The average denominator becomes small.
Sources and transparency
The formulas and examples were evidence-checked on 2026-09-10. This is educational mathematical content. It does not replace a field-specific standard, accounting policy, laboratory method, contractual definition or professional judgment.
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