What Is AQL and Acceptance Sampling?
Also known as: AQL, acceptable quality limit, acceptance quality limit, ANSI Z1.4, ISO 2859-1, sampling plan
Definition
Acceptance sampling inspects a random sample from a lot and accepts or rejects the whole lot based on the number of defects found. AQL (acceptance quality limit) is the worst defect rate considered acceptable as a process average in that plan.
AQL and Acceptance Sampling Explained
Checking every piece is slow, costly and, for visual work, not even reliable. A sampling plan picks a sample size n from a lot of size N and an accept number c. If the sample has c or fewer defects the lot is accepted, otherwise it is rejected or screened. The tables in ANSI/ASQ Z1.4 (equivalent to ISO 2859-1) and ISO 2859 family, and MIL-STD-105E before them, give plans by lot size, inspection level and AQL.
The procedure: choose the inspection level (general level II is the default, levels I and III give less or more discrimination), find the sample size code letter from lot size, then read n, accept and reject numbers for the chosen AQL. Typical AQLs are 0.65 for major defects and 2.5 or 4.0 for minor ones, with critical defects usually 0 tolerated. Switching rules move you between normal, tightened and reduced inspection based on recent lot history.
Sampling has limits. An AQL of 1.0 does not mean the shipped product is 1 percent defective in every lot, and an accepted lot can still contain defects. The operating characteristic (OC) curve shows the probability of acceptance at each true defect rate. For safety critical parts, or when a process is capable, 100 percent inspection, skip-lot arrangements or SPC-based acceptance are usual alternatives, and zero-accept plans (c=0) are common in aerospace and medical.
In ERP receiving inspection, the sampling plan is attached to the item, supplier or inspection plan. SyteLine and LN let you define inspection plans with sample size rules and acceptance criteria at receipt or in process, and the ERP then holds the lot until the inspector records the result and the lot is accepted, rejected or sent to the MRB.
Why It Matters
- It balances inspection cost against the risk of accepting bad lots, which matters when inspecting every unit is impractical or destructive.
- Customer and supplier agreements often define AQL in purchase terms, so disputes are settled by the agreed plan rather than opinion.
- A well tuned plan, with tightened inspection after failures, gives suppliers a reason to improve.
- Wrong assumptions about AQL lead to shipping defects that the contract allowed but the customer did not expect.
In Practice
A lot of 1,200 molded housings arrives. Using ANSI Z1.4 general level II, code letter L gives a sample of 200. At AQL 1.0 for major defects the plan accepts at 5 and rejects at 6. The inspector finds 4 cracked housings, so the lot is accepted, but the supplier scorecard logs a 2 percent sample defect rate. If the next lot fails, switching rules move the supplier to tightened inspection.
Frequently Asked Questions
What does AQL 2.5 mean?
It is the maximum defect rate, 2.5 percent, treated as acceptable as a process average in the sampling scheme. Individual accepted lots can contain more or fewer defects than that.
What is the difference between AQL and LTPD?
AQL is the quality level the plan accepts most of the time. LTPD (lot tolerance percent defective) is the poor quality level the plan rejects most of the time, protecting the buyer.
What is a zero acceptance number plan?
A plan where the lot is rejected if any defect is found in the sample (c=0). It is common for critical characteristics in aerospace and medical devices.
Can ERP automate sampling?
Yes, most ERP quality modules can assign a sample size rule per item or supplier at receipt and hold the lot until the result is entered, although the sampling tables themselves are set up by quality.
Related Terms
Incoming Inspection (Receiving Inspection)
Incoming inspection, also called receiving inspection or IQC, verifies purchased material against the purchase order, drawing and specification before it is released to stock or production. Checks include documents, count, visual, dimensional and sometimes lab testing.
SPC (Statistical Process Control)
SPC (Statistical Process Control) is the use of control charts and statistical rules to monitor a process in real time, distinguishing normal common-cause variation from special-cause variation that signals a genuine change requiring action.
First Pass Yield
First pass yield is the percentage of units that complete a process correctly the first time, without rework, repair, or retest. Units that eventually pass after rework are excluded, so the metric captures hidden factory loss.
Non-Conformance Report (NCR)
A non-conformance report (NCR) is the formal record that a part, material, process or document failed a specified requirement. It captures what failed, how many, where, and the disposition decision (use as is, rework, repair, scrap or return to supplier).
Cp and Cpk
Cp and Cpk are process capability indices. Cp compares the specification width to the process spread, assuming perfect centering, while Cpk also accounts for how far the process mean has drifted from the center of the tolerance.
Go Deeper
SyteLine Receiving and Inspection Setup
Configure SyteLine receiving and quality inspection workflows. Set up PO receiving, inspection plans, and non-conformance triggers in CloudSuite Industrial ERP.
SyteLine Inspection Plan Configuration
Configure SyteLine inspection plans with test criteria, sampling rules, and measurement specs. Set up receiving, in-process, and final inspection plans in CloudSuite.
SyteLine Supplier Quality Management
Configure SyteLine supplier quality management for vendor quality scoring, incoming inspection tracking, and supplier corrective actions in CloudSuite Industrial.
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