Comparing measurement uncertainty values

Estimates of measurement uncertainty (MU) are now ubiquitous in analytical chemistry. Having sufficiently reliable estimates is important for decision making, e.g., deciding whether a particular measurement method produces results that are fit for the intended purpose (FFP). In some situations it can be useful to compare these estimates. For example, we may wish to establish whether the MU for an in situ method, where measurements are made directly in the field, is significantly different (one would often expect it to be larger) from that obtained using a more traditional laboratory method. Or we might want to compare the different components of MU (e.g., compare the uncertainty arising from the sampling activity with the uncertainty from the analytical method) thus enabling us to take a cost-effective approach to reducing the overall, or combined MU. Quoted values of MU are only ever estimates however, being subject to their own uncertainties (AMCTB No. 105). This has implications when two values of MU are compared. An example is provided where the sampling and analytical components of MU are compared for measurements of the nitrate concentration in a field of lettuces. It is shown that in this case it would be more cost effective to reduce the sampling component of MU in order to reduce the overall MU.