The type of data treatment is different depending on whether the comparison, in
particular a key comparison of the MRA (mutual recognition agreement), is of the
hierarchical or non-hierarchical type. This term does not mean a possible hierarchy among
the participant laboratories; nor, in the opposite sense, a non-hierarchy among them like
in the MRA key comparisons, but an intrinsic characteristic of the comparison measurand or
design. It is a typical hierarchical comparison when the comparison
involves artefact standards. In this case, the summary parameters of the comparison are
hierarchically higher than the input dataset. In case of non-hierarchical
comparisons, the summary parameters are generally not of a hierarchically
higher level than the input dataset, because the comparison dataset can be considered
drawn from a single super-population. This happens, when a single standard is circulated
for measurement; when the measured samples are all drawn from a single batch of a
reference material; when the standards are all realisations of a single condition – namely
a physical or chemical state. This paper will discuss in detail these two categories.