An interlaboratory study of heavy metal content in compost

Heavy metals in 15 composts were determined by four Italian laboratories participating in a study involving interlaboratory work. The data obtained were processed using a statistical approach focused on measuring precision, that is to say, on repeatability (r) and reproducibility (R) limit determination. The results indicate how precision is substantially in agreement with what is reported in the literature, and that repeatability and reproducibility affect the statistical validity of data. It has been shown that only for data with R ≤ 1.25 r can the means of four laboratories be considered as being of the same population (the same compost), revealed by ANOVA as such. For each heavy metal, consideration was given only to those results where R ≤ 1.25 r in order to determine the interval of confidence. Upper variation limits (expressed as relative variation with respect to the mean) were then used to calculate the upper value to be detected to respect heavy metal limits according to Italian law.