Alternative designs for the assessment of homogeneity: use of characterisation data

In order to reduce analysis cost and save samples, the homogeneity study of two titanium-certified reference materials was combined with the interlaboratory characterisation of that material. Samples were selected using a random-stratified sampling scheme and were sent to nine and 11 laboratories, respectively. The data were normalised to screen for trends in the production sequence, outliers and normality of the distribution of sample means. The non-normalised data were screened for outlying variances, and the uncertainty of homogeneity was evaluated using two-way analysis of variance. The uncertainty contributions determined were in all cases significant, but did usually not dominate the uncertainty of the assigned value. The findings were confirmed by a homogeneity study with a limited scope on one of the two materials. While the assessment of homogeneity using the approach was successful, the goal of reducing the consumption of samples and reducing analytical cost was only partially realised: a similar number of samples were consumed and 5 % – 10 % lower analytical cost than in a classical, separate homogeneity and characterisation study set-up was incurred.