ISO 3951-3:2007

Overview

ISO 3951-3:2007 - Sampling procedures for inspection by variables - Part 3 describes an acceptance sampling system of double sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection. The standard specifies how to decide lot acceptability from one or two random samples by estimating the percentage nonconforming (items outside specified limits). Its objectives are to give a high probability of acceptance for acceptable lots and strong protection against inferior lots through automatic switching rules (normal, tightened, reduced inspection).

Key topics and requirements

• Double sampling by variables: Inspect a first random sample; accept or reject immediately if results are decisive, otherwise draw a second sample and combine results for a final decision.
• Indexed by AQL: Plans are referenced to the Acceptance Quality Limit (AQL) to align operating characteristics with consumer/producer risk expectations.
• Methods for process variance:
  • “s” method - process standard deviation unknown (estimate from sample).
  • “σ” method - process standard deviation known or well estimated.
• Switching rules: Automatic switching to tightened inspection or discontinuation on quality deterioration; possible switch to reduced inspection when quality is consistently good.
• Assumptions / applicability conditions:
  • Continuing series of lots from a single producer/process.
  • Single quality characteristic measured on a continuous scale.
  • Measurement error negligible (≤ 10% of process SD).
  • Production stable (statistical control) and approximately normally distributed.
  • Administrative acceptance of a potential second sample.
  • A contract/standard defines upper and/or lower specification limits.
• Limitations: Not suitable for lots already screened for nonconforming items.

Practical applications

ISO 3951-3 is practical for quality and inspection programs where more efficient variable-data sampling is desired compared with attribute plans. Typical users:

• Quality engineers and statisticians designing lot acceptance procedures.
• Purchasing and inspection authorities specifying acceptance criteria in contracts.
• Manufacturers seeking to reduce inspection cost while maintaining consumer protection.
• Third-party inspectors implementing lot-by-lot variable sampling.

Benefits:

• Substantial inspection effort savings (first-sample size often ~40% smaller than corresponding single-sample plans).
• Fast decisions when quality is very good or poor; second sample only needed when results are equivocal.
• Clear framework for switching between inspection levels to balance consumer protection and inspection cost.

Related standards

• ISO 3951 parts 1, 2 and 5 (single sampling, multi-characteristic, sequential plans).
• ISO 2859-1 - attribute-based sampling plans (ISO 3951-3 is complementary and aligned in operating characteristics).