Tritium Urinalysis Intercomparison - May 2007

Appendix A

Relative Bias :

Relative bias is an indicator of how closely a reported measurement corresponds to the known tritium 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 the same day. The result obtained is represented by y_jkl where:

j=the laboratory; j=1, 2, ..., n

k=the spike; k=1, 2, ..., 3

and l=the aliquot; 1=1, 2, ..., 5

The relative bias of the result for each aliquot (b_jkl) of a particular spike is calculated thus:

where a_k=the known value for a particular spike.

The relative bias (b_jk) for the mean of measurements on all the aliquots of a given spike is then obtained as follows:

where L=the number of replicate measurements (=5, in this intercomparison).

Relative Precision :

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 this report. It is calculated as follows: