TS ISO 3951-3

TS ISO 3951-3 addresses double sampling schemes for acceptance sampling by variables in lot-by-lot inspection, indexed by the acceptance quality limit (AQL). It defines an acceptance sampling system for inspection based on measured characteristics, so the decision about a lot is made from quantitative data rather than from counting conforming and nonconforming items alone.

What inspection by variables means

Inspection by variables uses measured results and their variability, instead of only classifying items as conforming or nonconforming. In general technical practice, this is useful for characteristics such as dimensions, mass, strength, or other continuous properties. AQL-indexed schemes then provide a structured way to select sampling plans against a reference quality level. That gives manufacturers and inspectors a common statistical basis for decision-making, while the standard’s actual scope remains confined to the acceptance sampling system described in the title and abstract.

How double sampling schemes work

Double sampling uses two stages of sampling rather than relying on a single sample. If the first sample does not lead to an immediate accept/reject decision, a second sample is taken and evaluated according to the pre-established rules. This creates a flexible lot-by-lot inspection method that can reduce unnecessary decisions when the evidence is not yet conclusive. In practical quality control, the sample size, decision limits, and acceptance logic are linked, and this standard concerns the definition of that system for variables-based inspection.

In broader industrial use, such schemes support statistically consistent acceptance decisions in incoming inspection, supplier control, and process verification whenever measured characteristics are the right basis for judgment.