ISO 17994:2014

Overview

ISO 17994:2014 - Water quality specifies a statistical and procedural framework for comparing the relative recovery (average quantitative performance) of two microbiological methods that target the same group or species of microorganisms. The standard covers methods based on colony counts or most probable number (MPN) estimates and provides the mathematical basis - rooted in paired t‑test principles - for evaluating the mean relative difference between paired confirmed counts. ISO 17994:2014 requires that both methods already have established performance characteristics (see ISO/TR 13843) and clarifies decision rules based on confidence intervals and predetermined stipulated limits.

Key technical topics and requirements

• Scope and principle
  • Compares two quantitative microbiological methods (one may be a reference or standard method).
  • Uses paired confirmed counts (one determination per method on equal portions of the same mixed sample).
  • Statistical evaluation is based on the paired t‑test and relative differences on a natural logarithmic scale.
• Data types
  • Supports colony counts and MPN estimates; distinguishes presumptive and confirmed counts and uses confirmation coefficients.
• Sampling and study design
  • Emphasizes a wide range of representative samples (natural or prepared) and recommends multi‑laboratory participation for broader validity; single‑lab studies can be valid if sample diversity is adequate.
  • Requires samples with sufficient target organism density to avoid excessive zero counts.
• Statistical calculations
  • Preliminary data editing, calculation of basic relative differences, experimental standard deviation, and half‑width of confidence intervals.
  • Decision rules for “not‑different” vs. different methods using two‑sided and one‑sided evaluations and a predetermined stipulated limit (confidence interval bounds ±2L). Guidance notes suggest commonly used 2L limits (e.g., ~10% for drinking water; ~20% for some environmental waters), though those are illustrative, not prescriptive.
• Reporting
  • Specifies elements required in a test report and includes informative annexes (flowchart, worked examples, sample‑size derivation).

Practical applications

• Validating and comparing alternative or new microbiological methods against established methods.
• Designing interlaboratory comparison trials and method comparison studies for water laboratories.
• Supporting method acceptance by regulators, standard developers, method manufacturers, and environmental monitoring programs.
• Informing decisions on method equivalence where average recovery and bias are critical (e.g., drinking water compliance, bathing‑water monitoring).

Who should use this standard

• Water and environmental microbiology laboratories
• Method validation specialists and quality assurance managers
• Regulators and accreditation bodies
• Diagnostic and test‑kit manufacturers
• Standards developers conducting method comparisons

Related standards

• ISO/TR 13843 - Guidance on validation of microbiological methods (performance characteristics required prior to comparison)
• ISO 8199 - General guidance on enumeration of microorganisms by culture

Keywords: ISO 17994:2014, water quality standard, comparison of microbiological methods, relative recovery, quantitative methods, paired t‑test, confirmed counts, MPN, method validation, ISO/TR 13843.