Relative bias is used as the indicator of how closely a reported measurement corresponds to the known radionuclide activity of a urine sample. It is calculated in the following manner. First, each observation for a spiked sample is corrected for the appropriate urine blank analyzed on the same day. The result obtained is represented by yjkl where:
j= the laboratory; j=1, 2, ..., n
k= the spike
and l= the aliquot; 1=1, 2, ..., 5
The relative bias of the result for each aliquot (bjkl) of a particular spike is calculated thus:
where ak= the known value for a particular spike.
The mean relative bias (bjk) for the measurements on all the aliquots of a given spike is then obtained as follows:
where L= the number of replicate measurements (=3, in this intercomparison).
In this program, the standard deviation of the bias is taken as the measure of the reproducibility of an analysis, and is referred to as "relative precision" in the reports. It is calculated as follows: