ISO 5667-20:2008

Overview - ISO 5667-20:2008 (Water quality - Sampling - Part 20)

ISO 5667-20:2008 provides guidance on using water quality sampling data for decision making, with a focus on compliance with thresholds and classification systems. The standard sets out principles, basic requirements and illustrative methods to assess the confidence that monitored water quality (a) meets targets or regulatory thresholds, (b) has changed over time, or (c) falls into a particular class in a grading system. It emphasises the need to quantify and account for sampling uncertainty and overall uncertainty (including analytical and handling errors) when drawing management or regulatory conclusions.

Key topics and technical requirements

• Assessment of confidence: Methods to quantify how confident you can be that water meets targets, has changed, or belongs to a class.
• Types of error and variation: Discussion of analytical error, sampling error and overall uncertainty that influence decision outcomes.
• Thresholds and percentiles: Guidance on the expression of thresholds and the recommendation that absolute limits be treated as percentiles for compliance assessment to accommodate sampling variability.
• Preliminary sensitivity analysis: Procedures for testing how decisions respond to error and uncertainty (note: the standard does not cover the full range of statistical techniques).
• Methods and illustrative examples: Worked approaches for estimating summary statistics, confidence of failure, non‑parametric methods and percentile-based thresholds.
• Recommendations for sampling programmes: General advice on the form and scale of sampling programmes so that sampling design is compatible with decision‑making needs.
• Relationship to other guidance: Part of the ISO 5667 series on water sampling; references concepts such as total assay error (ISO/IEC Guide 99).

Practical applications - who uses this standard

• Environmental regulators and permitting authorities assessing legal compliance of discharges and water bodies.
• Water utilities and wastewater operators monitoring performance against discharge limits.
• Environmental consultants and laboratories designing sampling programmes and interpreting monitoring results.
• River basin managers, policy makers and public‑health officials who classify water bodies or detect changes over time.
  Use cases include compliance reporting, classification of ecological status, early detection of water quality deterioration, and designing monitoring networks that support defensible decisions.

Related standards

• ISO 5667 series (Parts 1–19, 21–23) - guidance on sampling design, preservation, and specific media (rivers, groundwater, sediments, wastewater, etc.).
• ISO/IEC Guide 99 - terminology for measurement uncertainty and total assay error (referenced conceptually).

Keywords: ISO 5667-20:2008, water quality sampling, sampling data for decision making, thresholds, classification systems, sampling uncertainty, overall uncertainty, compliance assessment, percentiles, confidence limits.