• Table of Content
• Foreword
• Scope
• Uncertainty
• Analytical Measurement
• The Process
• Step 1: Specification
• Step 2: Identification
• Step 3: Quantification
• Step 4: Calculation
• Reporting Uncertainty
• Appendix A: Examples
  • Example 1: Calibration Standard
  • Example 2: Standardising NaOH
  • Example 3: An Acid/Base Titration
  • Example 4: In-house Validation Studies
  • Example 5: Determ. Cadmium Release
  • Example 6: Crude Fibre in Animal Feed
  • Example 7: Lead in Water - double IDMS
• Appendix B: Definitions
• Appendix C: Analytical Proc.
• Appendix D: Analyse U Sources
• Appendix E: Statistics
• Appendix F: MU at the Limit
• Appendix G: Common Sources
• Appendix H: Bibliography

4. The Process of Measurement Uncertainty Estimation

4.1. Uncertainty estimation is simple in principle. The following paragraphs summarise the tasks that need to be performed in order to obtain an estimate of the uncertainty associated with a measurement result. Subsequent chapters provide additional guidance applicable in different circumstances, particularly relating to the use of data from method validation studies and the use of formal uncertainty propagation principles. The steps involved are:

Step 1: Specify Measurand
Write down a clear statement of what is being measured, including the relationship between the measurand and the parameters (e.g. measured quantities, constants, calibration standards etc.) upon which it depends. Where possible, include corrections for known systematic effects. The specification information should be given in the relevant Standard Operating Procedure (SOP) or other method description.

Step 2: Identify Uncertainty Sources
List the possible sources of uncertainty. This will include sources that contribute to the uncertainty on the parameters in the relationship specified in step 1, but may include other sources and must include sources arising from chemical assumptions. A general procedure for forming a structured list is suggested at appendix d.

Step 3: Quantify Uncertainty Components
Measure or estimate the size of the uncertainty component associated with each potential source of uncertainty identified. It is often possible to estimate or determine a single contribution to uncertainty associated with a number of separate sources. It is also important to consider whether available data accounts sufficiently for all sources of uncertainty, and plan additional experiments and studies carefully to ensure that all sources of uncertainty are adequately accounted for.

Step 4: Calculate Combined Uncertainty
The information obtained in step 3 will consist of a number of quantified contributions to overall uncertainty, whether associated with individual sources or with the combined effects of several sources. The contributions have to be expressed as standard deviations, and combined according to the appropriate rules, to give a combined standard uncertainty. The appropriate coverage factor should be applied to give an expanded uncertainty.

Figure 1 shows the process schematically more...

4.2. The following chapters provide guidance on the execution of all the steps listed above and shows how the procedure may be simplified depending on the information that is available about the combined effect of a number of sources.