Overview
ISO 7870-3:2020 - "Control charts - Part 3: Acceptance control charts" provides guidance for designing and using acceptance control charts to make process acceptance decisions. The standard establishes general procedures for determining sample sizes, action (acceptance) limits and decision criteria, and explains when acceptance control charts are appropriate. It complements Shewhart-type control chart practice by combining elements of acceptance sampling with process control to judge whether a process is acceptable for continued production.
Key technical topics and requirements
- When to use: Only where the within-subgroup variation is in-control and estimated efficiently, and a high level of process capability already exists.
- Distribution assumptions: Typically applied when the process variable is normally distributed, but the method can be adapted to non‑normal distributions.
- Design elements: Procedures for selecting subgroup sample size (n), setting acceptance control limits (ACLs), and specifying decision criteria and operating characteristics (probability of acceptance).
- Plotting and interpretation: Guidance on plotting acceptance control charts and interpreting chart signals in the context of process acceptance rather than only product disposition.
- Preconditions: A preliminary Shewhart study (e.g., X–R or X–s charts) is required to verify inherent stability of within‑subgroup variability.
- Calculation procedures and examples: Detailed steps for selection of element pairs (APL and RPL), sampling frequency, and worked examples illustrating sample-size, action-limit, and decision‑rule determination.
- Supplementary material: Factors for acceptance control limits, modified acceptance control charts and nomographs (Annex A) for chart design.
- Normative references: Relies on statistical terminology and symbols from ISO 3534‑1 and ISO 3534‑2.
Practical applications
- Use ISO 7870-3:2020 when you want to accept a process based on the risk that a specified proportion of items will meet specifications - e.g., where slight shifts in process level are tolerable and costly over‑control should be avoided.
- Typical applications include manufacturing operations with batch-to-batch location shifts (material batches, tool wear), high‑capability processes in automotive, aerospace, medical device, pharmaceuticals, and precision components.
- The standard helps quality engineers and statisticians design control strategies that balance economy and assurance of product conformity.
Who should use this standard
- Quality/control engineers, SPC practitioners, process statisticians and auditors
- Manufacturing engineers and production managers seeking robust acceptance rules
- Consultants and trainers designing SPC programs and acceptance-control procedures
Related standards
- ISO 3534‑1 and ISO 3534‑2 (statistical vocabulary and symbols)
- ISO 7870 series (other parts on Shewhart control charts and related SPC practices)
Keywords: ISO 7870-3:2020, acceptance control charts, control charts, statistical process control, sample size, action limits, decision criteria, process capability, Shewhart.