TS ISO 3951-5

TS ISO 3951-5 is the part of the ISO 3951 series that addresses sequential sampling plans for lot-by-lot inspection by variables. It provides a system of plans indexed by acceptance quality limit (AQL) for situations where the standard deviation is known. In practical terms, this means the inspection decision is based on measured values and their statistical behavior, rather than on simple pass/fail counting alone.

How sequential inspection works

In a sequential plan, inspection proceeds step by step instead of relying on one fixed sample size from the outset. Each new measurement contributes to the acceptance, rejection, or continued sampling decision for the lot. This makes the method particularly useful where measurements are costly, time-consuming, or otherwise worth using efficiently. In general technical context, inspection by variables uses numerical data such as dimensions, strength, or concentration, and evaluates both the center and the spread of the process results.

Why AQL and known standard deviation matter

The title of this standard shows two key ideas: the plans are indexed by AQL, and they assume a known standard deviation. AQL is a statistical reference point commonly used to relate an inspection plan to an acceptable level of lot quality. Knowing the standard deviation means the decision rules are applied under a defined variability condition, which is important for variables-based sampling. TS ISO 3951-5 defines the system of sequential sampling plans for inspection by variables, while the related standards in the series provide the shared terminology and statistical foundations used alongside it.