Full Inspection vs. Sampling Inspection Decision — AQL Is Not the Only Answer

Abstract: Should incoming and process inspections be full or sampling inspections? Should AQL be set at 0.65 or 1.0? Should inspection level I or II be used? Many companies rely on experience or a client's verbal request to determine the inspection plan, lacking a systematic approach that is risk-oriented, data-driven, and linked to supplier performance. This article, within the framework of GB/T 2828.1 / ISO 2859-1, provides a decision tree for inspection planning, a complete table example, normal/tightened/reduced transfer rules, and three industry-specific calculation cases to help quality engineers allocate inspection resources more effectively.

1. A Real Dilemma: Full Inspection Does Not Guarantee Delivery

A Tier 2 fastener factory has 6 incoming quality control (IQC) inspectors and over 800 types of incoming materials. The quality director set the policy of "full inspection for critical components, sampling inspection for general components," resulting in an ever-growing list of critical components—every time R&D adds a special characteristic, it is included in the list, which grew from 80 items to 230 items in six months. The inspection station queue time reached 4 hours, causing production lines to halt due to material shortages, and procurement complained about the quality department holding up materials.

The finance department calculated the cost: for a batch of 30,000 M8 bolts, a full inspection of thread pass/fail and appearance requires 18 person-hours; however, the same supplier had a PPM of 120 for 24 consecutive batches and a process Cpk of 1.6—the marginal benefit of full inspection is extremely low.

Conversely, another plastic component factory uniformly used AQL 4.0 for "low-value auxiliary materials," resulting in a color difference in a batch of color masterbatches that led to 6,000 pieces being reworked—the sampling plan did not match the defect consequences.

The core issue in both factories is the same: inspection planning does not answer four questions—(1) How severe are the consequences of failure? (2) How reliable are the supplier and the process? (3) Where is the economic breakpoint between full and sampling inspections? (4) How can the plan be dynamically adjusted based on performance?

11.1.1 has defined the logic for the four inspection stations: IQC, IPQC, FQC, and OQC; 11.1.2 focuses on how to scientifically determine the sampling plan itself.

2. The General Framework for Inspection Planning: First, Grade, Then Decide

2.1 Material and Characteristic Risk Grading

It is recommended to use at least a two-dimensional matrix:

Example Rules:

• A class + Supplier level II → Tightened sampling or 100% automatic inspection of key dimensions
• A class + Supplier level I for 6 consecutive months → Can evaluate skipping batches or transferring to source inspection
• C class + Supplier level I → Normal level II sampling is sufficient

2.2 Full Inspection vs. Sampling Inspection: Decision Tree (Text Version)

Start
 ├─ Is the defect safety/critical/regulatory? → Yes → 100% inspection or 100% automatic inspection + traceability
 ├─ No → Is the supplier newly certified/yellow card? → Yes → Tightened plan or temporary full inspection
 ├─ No → Is the process Cpk ≥ 1.33 and SPC controlled? → No → Tightened/full inspection until capability is proven
 ├─ Yes → Can the inspection be automated at low cost? → Yes → 100% online inspection (not manual full inspection)
 └─ No → Sample according to AQL + transfer rules

Key Point: Full inspection is not "more responsible"; manual full inspection also has issues of missed inspections, fatigue, and consistency—when 100% automatic inspection is possible, it is superior to manual sampling inspection.

3. AQL, Inspection Levels, and Sampling Types — What the Standard Says

3.1 AQL Is Not a "Permissible Defect Rate Target"

AQL (Acceptable Quality Limit) in GB/T 2828.1 is a sampling plan index parameter used to determine the sample size n and acceptance number Ac, rejection number Re—it does not mean telling the supplier "you can have 1% defects."

For supplier contracts, use PPM targets, Cpk requirements, zero-defect characteristics, etc., and AQL is only used for sampling plan design.

3.2 Inspection Levels I / II / III

3.3 Normal, Tightened, and Reduced Inspection

The standard provides transfer rules (which must be written into the inspection work instruction):

Normal → Tightened (example conditions):

• Two consecutive batches are rejected under tightened conditions; or
• Two out of five consecutive batches are rejected under normal conditions (specific conditions depend on the adopted standard clauses)

Tightened → Suspension/Full Inspection: Five consecutive batches are rejected under tightened conditions → Supplier improvement, 100% inspection until five consecutive batches are accepted

Normal → Reduced: Ten consecutive batches are accepted under normal inspection and production is stable → Can be reduced (must be approved)

Common Mistakes in Companies: Always staying at "Normal II" and never tightening—leading to underperforming suppliers using the same lenient plan.

4. Calculation Case 1: IQC Count Sampling (Normal Inspection)

Conditions:

• Material: Plastic shell, batch size N = 2000 pieces
• Inspection: Appearance scratches (count type)
• Plan: GB/T 2828.1, general inspection level II, AQL = 1.0

Table Lookup Results (illustrative, refer to the current effective standard table):

• Sample size code → Sample n = 125
• Ac = 3, Re = 4 (i.e., ≤3 defects in 125 pieces accepted, ≥4 defects rejected)

On-site Execution:

1. Randomly sample 125 pieces (sampling method must be defined: random, multi-stack, multi-box)
2. Record the number of defects d
3. d ≤ 3 → Batch accepted; d ≥ 4 → Batch rejected, initiate CAR (9.2.3)

If Rejected: Issue an 8D to the supplier; for this batch: isolate, 100% re-inspect, or return—the handling of rejected batches must be defined in the procedure, not "rejected but accepted."

5. Calculation Case 2: Trigger for Tightened Inspection

Continuing from the previous example, the supplier's continuous performance:

According to the transfer rules → enter tightened inspection (larger n, smaller Ac under the same AQL). At this point, procurement and quality jointly interview the supplier, and IQC freezes the reduction of this material until improvement is verified.

Management Significance: The AQL plan is a dynamic contract, not a static table.

6. Strategies Beyond Standard AQL

6.1 C=0 Sampling Plan (Zero Acceptance)

High-consequence characteristics (safety components, key fit dimensions) often use C=0 (Ac=0)—the sample size is usually larger than the AQL plan with Ac≥1, but there is no gray area after rejection.

Applicable: Regulatory components, customer CSR specified, internal class A characteristics.

6.2 Skip-Lot

Prerequisite: Supplier level I, PPM meets standards for 12 months, no major nonconformities in second-party audits.

Example Rules:

• Inspect 1 out of every 5 batches (Skip-Lot 1/5)
• Any batch rejected → immediately resume batch-by-batch inspection and revoke skip-lot eligibility for 3 months

Requires written customer approval or contract authorization.

6.3 Source Inspection Transfer

If the supplier's outgoing inspection data is credible (laboratory accreditation, SPC online, regular second-party audits) → the host factory reduces IQC samples and strengthens:

• Data review (Cpk of each batch, inspection records)
• Surprise inspections
• Periodic destructive verification

Linked with 9.1.2 second-party audits and 6.3 SPC.

6.4 100% Automatic Inspection

Thread gauges, visual inspection, weight checking—when the cycle allows and the equipment GR&R is qualified, 100% automatic inspection is often more cost-effective and reliable than manual sampling inspection (see 12.2.3).

7. What Should the Inspection Planning Document Include?

One document per material family, suggested sections:

1. Scope and Referenced Standards (version of 2828.1, CSR)
2. Risk Grading (A/B/C judgment)
3. Inspection Items and Characteristic Types (count/measurement)
4. Sampling Plan (full/sampling inspection, AQL, level, Ac/Re or C=0)
5. Transfer Rules (triggers for normal/tightened/reduced)
6. Handling of Rejected Batches (isolation, re-inspection, return, escalation path for concessions 3.4.3)
7. MSA Requirements (6.2.1, measurement characteristics)
8. Record and Traceability Fields
9. Correspondence with CP/PFMEA Detection Controls

Audit Focus: Is the plan being followed on-site? Are rejected batches closed-looped? Is tightening triggered?

8. Interface with PFMEA and Control Plan

PFMEA detection controls (D) should be reflected in the inspection planning:

The "sample/frequency" column in the CP must be consistent with the IQC documents—the most common nonconformity in audits is that the CP specifies full inspection while IQC performs sampling inspection.

9. Common Misunderstandings and Countermeasures

10. Conclusion

Full inspection and sampling inspection are not a matter of stance but a function of risk, cost, capability, and supplier performance.

AQL is the language of sampling plans, not a substitute for quality targets. Using grading, table lookup, transfer rules, and data feedback to form a closed loop, inspection can transform from a "material holding department" into the early warning system for supply chain risks.

Next steps recommended: Select the top 20 incoming materials, write an inspection planning document for each, and use 3 months of data to verify whether tightening/reduction is triggered according to the rules—this is more valuable than another debate on "full inspection or sampling inspection."

Knowledge code: 11.1.2

Version: v20260528

Author: Quality Think Tank