ISO 3951-5:2006

Overview - ISO 3951-5:2006 (Sequential sampling plans, AQL, known σ)

ISO 3951-5:2006 specifies a system of sequential sampling plans indexed by Acceptance Quality Limit (AQL) for inspection by variables when the process standard deviation is known. The standard provides lot-by-lot schemes for a continuing series of lots and a preferred AQL series from 0.01% to 10% nonconforming. It is intended to reduce average sample sizes versus single-sample plans while providing consumer protection through switching rules and curtailment.

Keywords: ISO 3951-5, sequential sampling plans, AQL, inspection by variables, known standard deviation, sampling procedures, lot-by-lot inspection.

Key topics and technical requirements

• Scope and assumptions

  • Designed for a continuing series of lots from one producer and one stable production process.
  • Applies to a single measurable quality characteristic on a continuous scale.
  • Known standard deviation (σ) of the quality characteristic is required.
  • Measurement uncertainty must be negligible relative to process σ.
  • Characteristic distribution must be normal (or transformable to normal).

• Sequential plan structure

  • Plans are indexed by AQL (preferred series 0.01–10 % nonconforming).
  • Include switching rules for normal, tightened, and reduced inspection to protect consumers and reward consistent quality.
  • Curtailment rule: an upper limit on cumulative sample size (introduced to limit excessively long inspections); ISO 3951-5 uses curtailment (e.g., up to 1.5 × n where n is corresponding single-sample size) as part of the plan design.
  • Acceptance/rejection decisions are based on statistics derived from measured values and the known σ.

• Limitations

  • Not suitable for previously screened lots or isolated/short series of lots.
  • Assumes production is under statistical control.
  • Intended for continuous measurements, not attribute-only sampling.

Applications and who uses it

• Primary users
  • Quality engineers, QA/QC managers, industrial statisticians, acceptance sampling practitioners, and contract/compliance managers.
• Practical uses
  • Reducing inspection cost/time for high-quality processes where testing individual items is expensive.
  • Implementing statistically rigorous lot acceptance based on measured characteristics (e.g., dimensions, weight, chemical concentration) when σ is known.
  • Procurement and supplier contracts that specify AQL-based variable inspection procedures.
• When to choose ISO 3951-5
  • When average sample size reduction is important and inspection logistics allow variable-time, sequential testing.
  • When production is stable and measurement systems are precise relative to process variability.

Related standards

• ISO 3951-1 - single sampling plans by variables (AQL)
• ISO 2859-1 - sampling by attributes (AQL) for lot-by-lot inspection
• ISO 3534-1 / ISO 3534-2 - statistical vocabulary and symbols

Use ISO 3951-5 when you need statistically efficient, AQL-indexed sequential sampling for measurable continuous characteristics with a known standard deviation.