A Relative percent difference (RPD) is a measure of precision, calculated by the following formula:

A Rd % = [X1 is X2]/Xave x 100 

X1 is the concentration observed with the first detector or equipment;
X2 = concentration observed with the second detector, equipment, or absolute value; and
Xave = average concentration = ((X1 + X2) / 2)

The relative percent difference (RPD) and coefficient of variation (COV) provide a measure of precision, but they are not equal. Below are example duplicate radon results and the corresponding values of relative percent difference and coefficient of variation:

Rn1

(pCi/L)
Rn2

(pCi/L)
RPD


(\%)
COV

(\%)
8 9 12 8
13 15 14 10
17 20 16 11
26 30 14 10
7.5 10 29 20

 

 

 

 

 

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