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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