Measurement Uncertainty
Definition
A quantitative statement of the range within which the true value of a measurement lies, at a stated confidence level (typically 95%, k=2). Required for all quantitative results under ISO/IEC 17025:2017 Clause 7.6. Calculated by combining random error (precision RSD) with systematic error (accuracy/trueness) and calibration contributions.
- Standard confidence level
- 95% (k=2), as required by ISO/IEC 17025:2017
- Required by
- ISO/IEC 17025:2017 Clause 7.6 for accredited forensic labs
- Reported as
- Plus-or-minus range or confidence interval around the measured value
Common questions
What is measurement uncertainty in forensic testing?+
Measurement uncertainty is the quantified range around a reported result within which the true value plausibly lies. It accounts for all known sources of error, including instrument precision, calibration standards, replicate variability, and analyst technique. Labs report it as a plus-or-minus value or confidence interval.
Why do forensic labs have to report measurement uncertainty?+
ISO 17025 requires accredited labs to estimate and state measurement uncertainty for all quantitative results. It tells courts and users how much confidence to place in a measurement. Without it, numbers look artificially precise.
How is measurement uncertainty calculated?+
Labs combine random error (precision, measured by relative standard deviation) with systematic error (accuracy and trueness) and calibration contributions. The result is typically reported at a 95% confidence level, meaning that 95% of intervals constructed this way will contain the true value.
Related terms
- Method Validation
- The process of demonstrating that an analytical method does what it claims: that it correctly identifies the target analyte, at what sensitivity,...
- Accreditation
- Formal recognition by a standards body (such as UKAS in the UK, A2LA in the US, or NABL in India) that a...
- Base-Rate Fallacy
- Ignoring the background frequency (base rate) of a characteristic when interpreting a match. A test specific to 1 in 10,000 people still...
- Corrective Action and CAPA
- The ISO 17025 Clause 8.7 requirement for root-cause analysis and documented corrective and preventive action (CAPA) following any nonconformity (PT failure, IQC...
- ENFSI (European Network of Forensic Science Institutes)
- The umbrella organisation representing public forensic science laboratories across Europe. Develops best practice manuals for forensic disciplines, runs proficiency testing schemes, and...
- Error Rate
- One of the Daubert factors: the known or potential rate at which a method produces false positives or false negatives, and whether...
- ILAC
- The International Laboratory Accreditation Cooperation, the global network of accreditation bodies. ILAC's Mutual Recognition Arrangement (MRA) means a result produced by an...
- ILAC MRA (International Laboratory Accreditation Cooperation Mutual Recognition Arrangement)
- An agreement among national accreditation bodies (NABL, A2LA, UKAS, DAkkS, etc.) under which accreditation by any signatory body is recognised as equivalent,...
- ISO/IEC 17025
- The international standard for testing and calibration laboratories, published jointly by the International Organization for Standardization and the International Electrotechnical Commission. It...
- ISO/IEC 17025:2017
- The international standard specifying general requirements for the competence of testing and calibration laboratories. Covers management (document control, non-conforming work, audits) and...
- Likelihood Ratio (LR)
- The ratio of two conditional probabilities: the probability of the observed evidence given the prosecution's hypothesis (same source), divided by the probability...
- Management Review
- The annual governance meeting required under ISO 17025 Clause 8.9, at which laboratory management reviews the aggregated quality performance data (PT results,...
Explained in these topics
- Error, Uncertainty and the Statistics of Forensic ScienceThe range around a reported value within which the true quantity plausibly lies, derived from instrument precision, calibration standards, replicate variabilit...
- ISO 17025, NABL, ENFSI and Quality Systems for Forensic Chemistry LaboratoriesA quantitative statement of the range within which the true value of a measurement lies, at a stated confidence level (typically 95%, k=2). Required for all qu...
- Quality Systems: ISO 17025 and AccreditationThe quantified range within which the true value of a measured quantity lies, accounting for all known sources of error in the measurement process. ISO 17025 r...