If you are using a quantitative test method where the estimation of analytical balance uncertainity is not specified, then this SOP from the US Navy Labs will be helpful. In summary, it is a 4-step process that involves:
a) Specifying the analyte of interest
b) Identifying the sources of uncertainty in analytical measurement
c) Quantifying the components
d) Calculating the combined and expanded analytical measurement uncertainty
For further information visit www.navylabs.navy.mil.