Performance Evaluation of Ground Water Reference Material by Statistical Methods

Abstract

As part of the Danish ground water monitoring program a multicomponent reference material for laboratory use has been developed. After the certification procedure was completed, further analysis was undertaken to investigate any possible effect of the analytical methods used by the laboratories. In the present case, a group of similar compounds were selected for the statistical analyses. Data was imbalanced, therefore a mixed model approach has been applied using the MIXED procedure of SASâ. Method effect was significant in two cases out of six. Some of the data sets caused problems with non positive definite G-matrices, probably because of too sparse data.