Expression of uncertainty in measurement: combining standard uncertainties

In the previous article in this series (November 2017 issue, page 19) we discussed how to calculate imprecision uncertainty as the determinant of measurement uncertainty (MU). However, when other contributory factors are identified we need a method of combining and defining the relationships between them. Our assay modelling (May 2017 issue, page 24) can help us produce this combined uncertainty for each measurand. In addition, combined uncertainties can themselves be used in other measurand results. Results may be calculated by combining many measurands, each with an individual standard or combined uncertainty.  Propagation of uncertainty explains how each standard uncertainty contributes to the overall uncertainty.  

For the time being we consider assays where all of the input contributors are measured using the same units. In doing so, it helps to keep things simple in the first instance. In the next article we will walk through the processes involved when we have many contributors, all with different sources of uncertainty, methods of quantification, magnitudes, correlation and units of measurement.

By the end of the next two articles we will have covered all approaches that should allow us to be able to produce an uncertainty budget for any quantitative assay we perform in laboratory pathology.