How to calculate combined uncertainty for NABL/ISO/IEC 17025:2017


To calculate the uncertainty of your measurements, you'll need to find the best estimate of your measurement and consider the results when you add or subtract the measurement of uncertainty.


After measuring the individual uncertainty due to type A and type B, the combined uncertainty (Uc) is calculated as follows; Uc = √ (U12 + U12 + U12+……) Where; U1, U2, U3 etc are uncertainties calculated with same units       

Another method of combining uncertainty with different units of measure is fractional uncertainty.

1.     Identify the equation

2.     Calculate the fractional uncertainty for each contributor

3.     Calculate the combined uncertainty

4.     Convert the result to the desired unit of measure.

The quantities that affects the measurand may not have a direct one to one relationship with it. Indeed they may be entirely different unit altogether. For example a dimensional laboratory may use Steel gauge block for calibrating measuring tools. A significant influence quantity is temperature. Because the gauge blocks has a significant coefficient of expansion, there is an uncertainty arises in their length due to uncertainty in temperature units. In order to translate the temperature uncertainty into uncertainty in length units, it is necessary to know how sensitive the length of the gauge blocks is to temperature. In other words a sensitivity coefficient is required

Instead of being calculated from the function f, sensitivity coefficients ∂f / ∂xi are sometimes determined experimentally: one measures the change in Y produced by a change in a particular Xi while holding the remaining input quantities constant. In this case, the knowledge of the function f (or a portion of it when only several sensitivity coefficients are so determined) is accordingly reduced to an empirical first-order Taylor series expansion based on the measured sensitivity coefficients.




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