Switching Rules and a Rejected Incoming Lot
An electronics assembler receives connectors in lots of 20,000 from a long-standing supplier. Incoming inspection uses ANSI/ASQ Z1.4, general inspection level II, single sampling, AQL 0.65%, normal inspection; for this lot size the plan is n = 125, Ac = 2, Re = 3. The previous 12 lots were accepted with a total of 9 nonconforming connectors. Lot 13 was rejected with 4 nonconforming units out of 125. The supplier's quality manager argues the lot is 99.97% conforming and demands the lot be accepted, offering a price credit. Production needs the connectors for a build that starts tomorrow; there is no other certified source. The purchasing manager suggests moving to reduced inspection immediately because 'the supplier has a good record,' and the plant manager asks whether the rejected lot can be screened and used.
1. Under the standard's switching rules, what inspection state applies now, and is reduced inspection justified?
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Stay on normal inspection; reduced inspection is not justified.
Switching rules of this type work as follows:
- Normal → tightened: when 2 out of 5 or fewer consecutive lots are rejected. Here Lot 13 is the first rejection in the sequence, so one rejection does not trigger tightened inspection.
- Normal → reduced: requires 10 consecutive lots accepted on normal inspection with total defectives at or below the applicable limit, plus stable production and no recent quality problems. A rejected lot resets that accumulation, so the 12 prior acceptances no longer count. Even before the rejection, the 9 defectives across 12 lots needed checking against the cumulative limit for the plan, not assumed acceptable.
- Tightened → normal: after 5 consecutive lots accepted on tightened inspection.
A single rejection therefore means: continue normal inspection, and watch the next four lots closely, because a second rejection within that window forces tightened inspection.
2. How should the rejected lot be dispositioned given tomorrow's build, and what is the significance of Ac = 2, Re = 3?
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A rejected lot is not automatically scrap, but it is not released as received either.
Options, in order of preference:
- Return to supplier for credit or replacement - cleanest, but the build schedule may not allow it.
- 100% screening (rectifying inspection) by the assembler or supplier, removing nonconforming units, with records and traceability. Screen for the specific defect, using a method proven to detect it.
- Use under customer-approved deviation/concession, if the defect is non-safety and the risk is accepted in writing.
- Scrap the lot.
Ac = 2, Re = 3 means 4 defectives ended the sampling and rejected the lot; it says nothing about how many defectives remain. Sampling creates an estimate, not a sorting. Screening removes known defectives from the lot but does not 'upgrade' the supplier's process, so the screened lot must be identified for traceability, and the screening cost should be charged back. Using unscreened rejected material to protect a build schedule is a risk transfer to the customer.
3. Evaluate the supplier's '99.97% conforming' claim and the price-credit offer. What evidence should be required before reduced inspection or skip-lot is considered?
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The claim is a point estimate from one sample, not a demonstrated process level.
Four defectives in 125 gives roughly 3.2% observed, with a wide confidence interval; the supplier's 99.97% figure does not match the data. AQL 0.65% is the quality level with a defined, high probability of acceptance - it is not a defect target and it does not guarantee the lot quality. A price credit compensates cost, not fitness for use; a connector failure in the field is not offset by a discount.
Before reduced inspection or skip-lot:
- Supplier control charts and capability on the defect characteristic, with evidence of stability.
- 100% automated test or mistake-proofing data from the supplier, if feasible.
- Several consecutive clean lots under normal inspection, meeting the standard's switching criteria.
- A documented, approved skip-lot plan with periodic verification and a defined return-to-normal trigger.
Until then, normal inspection plus a supplier corrective action request on the root cause of the 4 defectives is the proportionate response.