One of the most commonly used estimates of precision is the relative percent difference or RPD between two samples. In typical sampling situations a second sample is collected under the same conditions, time and location and submitted to the laboratory. Once analysed, the RPD can be calculated for the two samples.

To calculate RPD, where C1 is the concentration or reported value for sample 1 and C2 is the value for sample 2.

It is expected that RPD values have grater variance when comparing field samples verses laboratory duplicates. RPD calculations are typically applicable when the concentrations of the two samples are ≥ 5x RDL.

In general, RPD values > 20% could indicate a possible problem. Values >50% usually indicate a problem either due to contamination or non-homogeneous sample representation.