Supplier Lot Disposition With a Capability Warning
A receiving inspection team must disposition a 24,000-piece lot of sealed electrical connectors. The purchase order invokes a single-sampling plan at AQL 0.65%, general inspection level II. The sample size is 315, and inspection finds four major nonconformities; no critical nonconformities are found. The supplier's last six lots averaged 0.12% defective, but the customer's incoming test reports 1.1% functional failures on the previous lot. A critical seating dimension shows Cpk 0.92 on the supplier's process study. The lot is packaged in three sublots that were molded on two cavities. The line stops in three days, expedited replacement material costs a 22% premium, and the supplier argues the result is 'sampling error' because the sample is small relative to the lot. The quality manager asks for a disposition and a supplier action plan before the next production meeting.
1. Interpret the sample result against the sampling plan and state the disposition. Show the decision logic, including the effect of the critical dimension.
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The plan's sample size is 315. For this lot size and AQL 0.65%, the single-sampling acceptance number is 3 and rejection is 4 for major nonconformities. Four observed major nonconformities exceed the acceptance number, so the lot is rejected on the sampling result alone. Do not let the supplier reframe this as 'sampling error': the acceptance number already incorporates sampling risk, and re-sampling until a pass is not a valid disposition. The critical seating dimension adds process evidence, not a second vote: Cpk 0.92 is below the usual 1.33 capability expectation, and the previous lot's 1.1% customer failure rate corroborates a real process problem rather than a one-off sample. No critical nonconformities were found, so the decision rests on the major count plus capability. Disposition: segregate and reject the lot; require 100% screening only if the business chooses to use the material, and then screen for the seating dimension and the observed major defect, with a tightened sampling plan and traceability by sublot and cavity.
2. The supplier claims sampling error. What analysis and supplier actions do you require before accepting future lots?
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Treat the sampling-error claim as a request for stratification, not as a rebuttal. Ask for results by sublot and molding cavity; if the four nonconformities cluster in one cavity or sublot, the process is unstable and the lot is not homogeneous. Check measurement repeatability and the customer's 1.1% failure analysis to confirm the defect mode. Require a supplier corrective-action report with root cause, containment, and verification; a revised process study showing Cpk at or above 1.33 on the seating dimension; 100% inspection or error-proofing for the critical dimension until capability is demonstrated; and tightened inspection for the next three lots, with source inspection if the risk is high. Also review whether the previous lot's escape should trigger a containment action at the customer. Accepting future lots should be conditional on data, not on the supplier's explanation.
3. What should the team do to keep the line running without accepting the lot?
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Keep the line running without accepting the lot by using the disposition options separately: reject the lot for production, but authorize a controlled 100% screening only for a limited quantity if the screening detects the known defect modes and the customer accepts the screening evidence. In parallel, expedite material from an alternate source, borrow stock from another site, or resequence production to build products that do not use this connector. Do not split the lot and sample only the 'good' sublot unless the sublots are independently identified and the sampling plan is applied to each; otherwise the rejection stands. Give the production meeting a dated decision: screened quantity available, replacement lead time, premium cost, and residual risk if the line runs short.