K. Schrammnielsen et al., PESTICIDE ANALYSIS IN-GROUND WATER - STATISTICAL EVALUATION OF CERTIFICATION DATA OF A MULTICOMPONENT REFERENCE MATERIAL, Fresenius' journal of analytical chemistry, 361(5), 1998, pp. 404-409
21 laboratories participated in the certification of a multicomponent
ampouled reference material containing 24 pesticides. This paper descr
ibes the statistical analysis of the unbalanced data set returned by t
he participants in the certification. The simplest method of certifica
tion is achieved when laboratories report all the required data. In pr
actice, however, with complex analytical methods such as multicomponen
t analyses of organic compounds, laboratories are not able to report a
ll the required data. The response for each determinand in the materia
l has been modelled in a three-factor nested model, For computational
comfort the model has been rephrased. into a general linear model. The
statistical problems with unbalanced data are discussed in relation t
o ISO 5725 and ISO Guide 35. Tests for homogeneity are exact and uninf
luenced by the unbalance in data. Unbalanced data cause dependencies b
etween mean squares above the second lowest level of the model, thus e
xact hypothesis testing is not possible at higher levels. Approximate
tests for variation between rounds and laboratories has been construct
ed by means of Satterthwaite's formula [I]. A graphical plot of devian
ces from certified values has been applied to illustrate laboratory pe
rformance.