Overview
ISO 11843-6:2025 - Capability of detection, Part 6 - defines a statistical methodology for determining the critical value and the minimum detectable value (MDV) when both signal and background follow a Poisson distribution. The standard uses the conventional normal approximation to the Poisson law (consistent with ISO 11843-3 and ISO 11843-4) to compute decision thresholds, and it discusses the accuracy of that approximation (Annex B).
Key topics and requirements
- Scope and assumptions
- Applicable when variations in background and signal are describable by the Poisson distribution.
- Uses the normal approximation to derive variances, critical values and MDVs.
- Decision criteria
- Defines the critical value (yC) for the response variable to control the probability of false detection (α, error of the first kind).
- Defines the MDV (gx) tied to the probability of failing to detect a true signal (β, error of the second kind).
- Computation and reporting
- Provides formulae for computing yC using standard-normal quantiles and observed means/variances (Clause 6).
- Includes guidance on estimating mean and variance under normal approximation (Annex A) and examples (Annex D).
- Specifies reporting requirements for capability assessments and applications (Clauses 7 and 8).
- Practical measurement constraints
- Both signal and background must be raw, unprocessed counts.
- Prefer longer single measurements over many short ones (improves normal approximation).
- Repeat measurements are required to estimate means reliably.
- Detector must operate in its linear counting range; number of channels and FWHM must match between blank and sample (Annex C).
Applications and users
- Targeted at laboratories and instrument operators using pulse-counting detectors and spectroscopy techniques where counts follow Poisson statistics:
- X-ray fluorescence (XRF), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS)
- Electron and ion spectroscopy (AES, SIMS), and mass spectrometry (GC–MS)
- Typical uses:
- Determining detection limits for hazardous-substance screening (e.g., RoHS testing).
- Environmental monitoring, contamination control, material analysis.
- Establishing decision thresholds in quality control and compliance testing.
- Users include metrology labs, analytical chemists, instrument manufacturers, and accreditation bodies.
Related standards
- ISO 11843 series (Parts 1–4) - general capability of detection principles and linear calibration cases.
- ISO 3534-1 - statistical vocabulary and symbols.
- ISO Guide 30 - reference materials terms.
Keywords: ISO 11843-6:2025, capability of detection, Poisson distribution, normal approximation, critical value, minimum detectable value, detection limit, pulse-counting, spectroscopy, XRF, XRD, GCMS.