ZINQAP now uses the ISO Guide 43 recommended z-scores as an indication of the performance of a participant laboratory for quantitative assays. Z-scores simply indicate the number of standard deviations a certain data point (test result) is away from the mean or target value. This score provides the lab and also its (potential) clients and accreditation bodies a good indication of its analytical competence. An advantage of the score is that it allows the participant laboratories to compare themselves against the target value and also against previous performance.
The formula for z scores is calculated as follows
zki = z score for laboratory i on sample k; i=1,…,n.; k=1,…,p
Rki = Result from laboratory i on sample k; i=1,…,n; k=1,…,p
Mk. = Robust mean of results on sample k.
Sk. = Robust standard deviation of results on sample k.
The interpretation of the z-scores is easy:
| zki | < 2: or -2 < zki < 2
the participants result differs from the ‘true value’ less than 2 times the reproducibility standard deviation of the corresponding internationally accepted test method. Such a result is mentioned: good or satisfactory (will occur with normally distributed results in about 95% of all cases)
2< | zki | < 3 or -3< zki =-2 or 2= zki <3
Questionable (will occur with normally distributed results in about 5% of all cases)
| zki | > 3 or zki =-3 or zki =3
Unsatisfactory (will occur with normally distributed results in about 0.3% of all cases)