Boxplot fences in proficiency testing

The box and whiskers plot (boxplot) has shown to be an effective data visualization tool in proficiency testing. The boxplot provides a quick visual summary of the considered data set . All the descriptive aspects of data reported in a round of a proficiency test can be studied with this plot. Location (through median and quartiles), spread (by means of the interquartile range), shape (position of the median in the box), and possible outliers (values beyond of the fences) are simultaneously observed with the boxplot. In this paper, we propose a modification of the inner fences of the boxplot, in such a way that they are in accordance with the approach based on the z score (standard ISO 13528) and therefore useful to recognize graphically questionable and unacceptable values in proficiency testing.