Overview
ISO 28593:2017 specifies an accept-zero, credit-based acceptance sampling system by attributes for controlling outgoing quality. The standard defines single sampling schemes for lot-by-lot inspection where a lot is accepted only if the inspected sample contains zero nonconforming items. Plans are driven by a user-selected average outgoing quality limit (AOQL) and the credit principle, which reduces sample sizes when a supplier demonstrates consistent good quality while protecting the customer if quality deteriorates.
Keywords: ISO 28593, acceptance sampling, accept-zero, AOQL, credit principle, lot-by-lot inspection, attributes.
Key topics and requirements
- Accept-zero sampling: No lot is accepted if the sample contains one or more nonconforming items.
- Average outgoing quality limit (AOQL): The user specifies an AOQL (upper limit on long-term average outgoing nonconforming rate). Sampling schemes ensure the expected AOQ will not exceed that AOQL in the long run.
- Credit principle: A running credit (K) accumulates by lot size for consecutive accepted lots. Higher credit generally reduces required sample sizes; credit resets to zero on non-acceptance.
- 100% inspection on certain non-accepted lots: If a lot is non-accepted when credit is zero, it must be 100% inspected and all conforming items allowed into the market - this preserves consumer protection.
- Applicability constraints:
- Series of lots from a single producer/process (apply separately per supplier).
- Quality characteristics must be classifiable as conforming/nonconforming (attribute inspection).
- Inspection must be non-destructive and classification error negligible.
- Operational notes: No restriction on AOQL value or successive lot sizes; sample sizes are integers and tend to be slightly conservative. For isolated or very short series of lots, realized AOQ can exceed AOQL though expected AOQ remains within limits.
Practical applications and users
ISO 28593 is practical where long-term control of outgoing quality is required with minimal inspection effort:
- Suppliers / Manufacturers - reduce inspection costs while incentivizing defect-free production through accumulated credit.
- Buyers / Quality Assurance Teams - enforce contract quality levels and set AOQLs to control market risk.
- Regulatory agencies - adopt for control programs where expected market quality must be limited and inspection resource efficiency is important.
- QC engineers and statisticians - design sampling plans, implement credit tracking and interpret AOQ/AOQL behavior.
Use cases: incoming acceptance of discrete items by attributes, serial production lines, supplier qualification programs, and regulatory product surveillance.
Related standards
ISO 28593 is part of the ISO 28590–28599 series on sampling by attributes. Related documents include:
For implementation, refer to the full ISO 28593:2017 text for sampling formulas, treatment rules and informative examples.
Open preview (PDF)Acceptance sampling procedures by attributes — Accept-zero sampling system based on credit principle for controlling outgoing quality
