[Encyclios]

Components of uncertainty

In general, measurement uncertainty includes numerous sources of uncertainty, each of which is called a “component of uncertainty”. Some components arise from effects of a systematic nature (e.g., components associated with corrections, or values assigned to measurement samples), and among these is definitional uncertainty. To estimate the overall uncertainty, it may be necessary to examine each component of uncertainty and treat it separately to assess its contribution to the total uncertainty. Most of the time, however, it is possible to evaluate the simultaneous effect of several components, which allows us to simplify the uncertainty calculation. For a measurement result y we may have:

• Combined standard uncertainty, \(u_c(y)\), is the total uncertainty of the measurement result \(y\); it is a mean square deviation estimated as the positive square root of the total variance obtained by combining all components of uncertainty.
• Expanded uncertainty, \(U(y)\), obtained by multiplying the previous \(u_c(y)\), by a coverage factor \(k\): provides a range within which the value of the measurand is found with a higher confidence level; should be used in most cases of measurements in analytical chemistry. For a 95% confidence level, the coverage factor \(k = 2\).