The design of FIRI included 3 pairs of duplicate samples: A and B (Kauri wood) near background, D and F (Belfast wood) around 50 pMC, and G and J (barley mash) at 111 pMC. Why include duplicates? Duplicates by their nature allow us to explore the within-lab variability and to assess whether the quoted errors are representative. We can also explore the differences as a function of the sample activity. In this section, we explore the differences between the duplicates. We also consider some different graphical presentations. First, we summarize the differences, then graphically explore the boxplot (to consider the distribution of differences), then a scatterplot of the duplicate pair (to show correlation and reproducibility), and finally, a measure of agreement plot (Bland and Altman 1999). The horizontal axis in this final plot is the mean of the duplicate pair and the vertical axis is the difference in the duplicate pair. Agreement between the pairs would result in the points being randomly scattered around the horizontal zero line.