Analytical Measurement Terminology in the Laboratory. Part 2: Uncertainty of Measurement

What Is Uncertainty of Measurement?

We have seen that to identify the sources of measurement errors, it is helpful to distinguish between systematic (determinate) and random (indeterminate) errors. However, it is becoming more usual to consider the sum of all the contributions that affect the accuracy of measurement results and not just those from these two types of error. This is known as the uncertainty of measurement (also called the measurement uncertainty or simply the uncertainty). In this context, one uses the term partial uncertainty for a particular step of an analysis procedure (e.g. sample preparation) and combined uncertainty for the sum of all the uncertainties.

This article is based mainly on the concept of uncertainty described in the ISO publication “Guide to the Expression of Uncertainty in Measurement (GUM)”, published in 1993, as well as in the revised edition of 1995. The guide was published under the guidance of several different organizations such as the International Standards Organization (ISO), the International Bureau of Weights and Measures (BIPM), the Organization for International and Legal Metrology (OIML) and the International Union of Pure and Applied Chemistry (IUPAC). In this guide, the uncertainty of measurement is described as follows:

“The uncertainty of the result of a measurement reflects the lack of exact knowledge of the value of the measurand. The result of a measurement after correction for recognized systematic effects is still only an estimate of the value of the measurand because of the uncertainty arising from random effects and from imperfect correction of the result for systematic effects. Note − The result of a measurement (after correction) can unknowingly be very close to the value of the measurand (and hence have negligible error) even though it may have a large uncertainty. Thus the uncertainty of the result of a measurement should not be confused with the remaining unknown error.”

It follows from the definition that the uncertainty of measurement is a quantitative measure of the quality of the particular measurement result. It allows the user to estimate the reliability of the measurement results. In simple terms, the uncertainty of measurement is the range of values within which the value of the quantity being measured (the measurand) is expected to lie with a stated level of confidence. It is not the same as error because to estimate the error the “true” value must be known. Although the term uncertainty of measurement appears to have become widely accepted in laboratory practice and in international standards, it is important to note that most analytical procedures refer to the uncertainty of the result rather than the uncertainty of measurement. The concept of the uncertainty of measurement concerns not only the uncertainty due to the measurement procedure itself (i.e. result uncertainty) but also the uncertainty in connection with the sample, sample preparation, environmental influences, experimental parameters, the analyst and the evaluation of the measured data.

What Are the Sources of Measurement Uncertainty?

Although the concept of the uncertainty of measurement does not distinguish between systematic and random errors of measurement, the sources remain the same. The possible causes of measurement uncertainty have already been discussed in detail in UserCom 29 Part 1. The most important of these are 

• influences of the procedure (often called method bias).
  
• instrumental influences, 
• sampling and sample preparation, 
• environmental influences, 
• experimental parameters,  
• evaluation methodology, 
• time-dependent interdependencies, 
• shortcomings of the analyst, and 
• gross errors

How Is the Measurement Uncertainty Determined?

An analysis of the uncertainty of a particular measurement should only be performed by qualified staff. Qualified means here that the person responsible has good theoretical knowledge and practical experience of all the individual steps involved in the analysis procedure.

The following procedure based on the CITAC/EURACHEM Guide 2000 is recommended for determining the uncertainty of measurement:

Conclusions 

A well-based and documented analysis of measurement uncertainty demonstrates a professional approach to measurement technology. The results obtained provide the analyst with valuable information about the reliability of the measurement results, measurement performance, and the possibilities for optimization of the measurement procedure. Strictly speaking, measurement results without information about the measurement uncertainty cannot be reliably interpreted. The estimation of measurement uncertainty is therefore an important step in the validation of an analytical procedure. 

_{Analytical Measurement Terminology in the Laboratory, Part 2: Uncertainty of Measurement | Thermal Analysis Application No. UC 301 | Application published in METTLER TOLEDO Thermal Analysis UserCom 30}