Sampling and Switching Rules in Software Module Incoming Inspection
A quality inspection service performs incoming inspection on software modules delivered by a supplier. Each monthly lot contains 40 modules. The contract specifies general inspection level II, single sampling, AQL 1.0% for major defects, and zero acceptance for critical defects such as security exposure or data loss. The service samples 8 modules per lot, consistent with the plan. In the last three lots, inspectors found two critical defects in the payment module of lot A, one critical defect in the reporting module of lot B, and no critical defects in lot C. The supplier's overall defect rate is 4.5% against a contractual target of 1.5%. The supplier argues that the findings are 'test-data issues' and asks that lot A be accepted because the defects were found before deployment. The service's procedure requires switching rules and supplier corrective action.
1. What disposition should be given to lots A, B and C, and what switching action is triggered?
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Reject lots A and B. The plan has zero acceptance for critical defects, so a single critical finding rejects the lot regardless of the major-defect count; lot A has two critical findings and lot B has one. Lot C can be accepted only if no critical defect was found and the major-defect sample result is within the acceptance number; the scenario states no critical defect in lot C, so accept subject to confirming the major-defect count. The switching rules are triggered because two lots were rejected on original inspection. Because the rejections involve critical defects, do not wait for the normal tightened-inspection trigger; move immediately to tightened inspection and 100% inspection of the payment and reporting modules until five consecutive lots are accepted on original inspection, then return to normal. Record the defect classification, the rejection dates and the switch date. The AQL of 1.0% applies to major defects, not critical defects; critical defects have zero acceptance and cannot be relaxed by the supplier's request.
2. How should the service evaluate the supplier's claim that the defects are 'test-data issues'?
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Treat the claim as a classification dispute and require objective evidence. The supplier must show that the defect cannot occur with production data, including the data path, validation rules and a reproducible counterexample. If the module accepts malformed data that can expose security or lose data, the origin of the data does not make the defect non-critical; the defect is in input validation or error handling. The service should keep the critical classification unless the supplier provides evidence and the quality manager approves a reclassification through a documented deviation. Do not reclassify after disposition to make the lot pass, because that would undermine the inspection's impartiality and the traceability of the lot decision. Open a supplier corrective action request with root cause, containment, verification and a deadline. If the supplier disputes the classification, escalate to the technical authority named in the inspection procedure rather than negotiating informally.
3. Should the sampling plan be revised, and if so, how?
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Yes. The current plan is weak for critical defects: with lot size 40 and level II, sample size 8, the probability of detecting a low-rate critical defect is modest, and critical defects are severe even when rare. Retain the tightened sampling for major defects, but add a critical-defect-focused approach: 100% automated scanning plus targeted manual inspection of security and data-loss paths for payment and reporting modules. Alternatively, increase the sample size for critical characteristics and justify it with the actual defect rate and required confidence. Recompute the plan using the observed 4.5% defect rate against the 1.5% target and document the rationale. Keep the zero-acceptance rule for critical defects. After five consecutive accepted lots, return to normal sampling, but maintain a trend chart and escalate if the defect rate exceeds 1.5% again. The plan revision must be agreed with the customer and recorded in the inspection procedure.