TDI-Brooks International - Scientific Services On A Global Basis

Accuracy (z-score) and Precision (p-score) Assessments

Accuracy Assessment (z-score)

Where x is the individual laboratory result, X is the "Exercise Assigned Value," and s is the target value for standard deviation.

As described in the IUPAC guidelines, the choice of s is dependent upon data quality objectives of particular program. It can be "fixed" and arrived at by perception, prescription, or reference to validated methodology (e.g., s = 0.125 X, X is the analyte concentration) or it can be an estimate of the actual variation (e.g., the calculated s from the exercise data). The "fixed" performance criterion is more useful in the comparison of a laboratory's performance on different materials while the use of the actual variation may be more useful within a given exercise, for example, if the determination of a particular analyte is more problematic than usual.

NIST has calculated and reported z-scores using both approaches for each analyte for each laboratory. At a previous workshop, it was decided to use "25% of the exercise assigned value" as the fixed target value for standard deviation for this program, at least for a few years. We also calculated z-scores based on "one assigned-value standard deviation." The z-scores calculated for these exercises can thus be interpreted as shown in the following examples:

z-score (25% X):
+1 -> laboratory result is 25% higher than the assigned value
-2 -> laboratory result is 50% lower than the assigned value

z-score (s):
+ 1 -> laboratory result is one "exercise standard deviation" higher than the assigned value
-2 -> laboratory result is "two exercise standard deviations" lower than the assigned value

From a scientific point of view, IUPAC does not recommend the classification of z-scores but allows that it is possible to classify scores, e.g.:

| z | <= 2 Satisfactory
2 < | z | < 3 Questionable
| z | => 3 Unsatisfactory

The NIST report shows the calculated z-scores for each laboratory for each reported analyte. These tables of the results and performance include a summary of the number of reported analytes that fall within each category for each laboratory. Figures show the distribution of z-scores (25%) by analyte.

Precision Assessment (p-score)

Since 1995, laboratories have been requested to process each replicate in a different sample set for precision assessment. For the calculation of p-scores for this program, the current target CV for the three replicates is 15%. Tables show the calculated p-scores for each laboratory for each reported analyte.