ISO 3951-2:2013

Overview

ISO 3951-2:2013 specifies an acceptance sampling system of single sampling plans for inspection by variables, indexed by the Acceptance Quality Limit (AQL). Intended for users already familiar with variable sampling, the standard provides technical rules for lot-by-lot inspection of independent quality characteristics in a continuing production series supplied by a single producer and process. It complements the more introductory ISO 3951-1 and relates to attribute-based schemes in ISO 2859-1.

Key topics and technical requirements

• Single sampling plans indexed by AQL: Plans are expressed in terms of AQL to balance producer and consumer protection.
• Variables inspection methods: Defines the s-method (sample standard deviation) and σ-method (process standard deviation) for univariate and multivariate characteristics.
• Acceptability criteria: Provides standard criteria formats (Form k and Form p*) to decide lot acceptance from variable measurements.
• Switching rules: Rules for normal, tightened and reduced inspection that:
  • Automatically tighten or discontinue inspection when quality deteriorates (consumer protection).
  • Allow reduction of inspection effort when consistent good quality is demonstrated (producer incentive).
• Risk and performance tools: Guidance and tables for consumer’s risk (CRQ), producer’s risk, Operating Characteristic (OC) curves, and related planning data.
• Measurement variability: Procedures for accommodating measurement error, normality assessment, and handling outliers.
• Planning and operation: Guidance on choosing inspection level, sample size, switching between s- and σ-methods, record keeping, and discontinuation/resumption of inspection.
• Normative annexes: Extensive tables and sample-plan annexes (e.g., sample size code letters, plan tables, values for MSSD/MPSD) to implement plans practically.

Practical applications and who should use it

ISO 3951-2 is applicable for:

• Quality engineers and inspection managers who implement lot-by-lot variable sampling in manufacturing.
• Suppliers and producers running a continuing series of lots for product acceptance decisions.
• Regulatory bodies and certification teams requiring technically robust variable-sampling procedures.
• Organizations needing to integrate AQL-indexed variable sampling into quality control systems to reduce inspection cost while maintaining consumer protection.

Typical applications include dimensional checks, functional measurement data, multivariate quality characteristics, and incoming inspection where characteristics are measured quantitatively.

Related standards

• ISO 3951-1 - introductory treatment for inspection by variables.
• ISO 2859-1 - attribute sampling plans; ISO 3951-2 notes similarities and differences to attribute-based AQL sampling.

Keywords: ISO 3951-2, sampling procedures, inspection by variables, AQL, single sampling plans, s-method, σ-method, switching rules, lot-by-lot inspection, OC curves.