Skip Lot Sampling Calculator
Skip lot sampling lets you inspect only a fraction of lots from a supplier with a long record of acceptance. This calculator shows the labor you would save and how hard qualification is at your supplier's actual acceptance rate.
Your numbers
ISO 2859-3 uses the fractions 1/2, 1/3, 1/4 and 1/5 for skip lot frequency.
Commonly 10 consecutive accepted lots under normal inspection; use the number in your procedure.
Your supplier's historical lot acceptance rate.
Your results
Skip lot procedures are defined in ISO 2859-3 and only apply to a supplier with a proven quality history, a controlled process and an approved quality system. Qualification counts, frequencies and requalification rules differ by procedure and customer. Use your own documented rules.
Chance of qualifying vs lot acceptance rate (%)
Sweeps Probability each lot is accepted from 80 to 100, holding your other inputs fixed.
Track supplier lot history and inspection status in your ERP
Optional: we can look at how lot acceptance history could drive skip lot status automatically at receipt.
Optional. The tool and every export work without this. No spam.
How to use the calculator
Enter the number of lots you receive per month, the skip frequency you are considering, the hours it takes to inspect one lot and your inspector cost. Then enter the number of consecutive accepted lots your procedure requires for qualification and your supplier's current lot acceptance rate. The results show lots inspected and skipped, hours and money saved, and the probability that the supplier gets through the qualification run without a rejection. The chart shows how sensitive that probability is to the acceptance rate.
- Use real acceptance history, not the supplier's own claim.
- Savings come from the skipped lots only; paperwork checks continue.
- Test several frequencies to see the trade-off.
What skip lot sampling is
In a skip lot procedure, such as ISO 2859-3, a supplier who has passed a qualification period under normal inspection moves to inspection of a fraction of lots, for example one in every three. The lots that are inspected still use the normal sampling plan. If an inspected lot is rejected, the supplier normally returns to lot by lot inspection and must requalify. The method is meant for products from a stable process and a supplier with a quality system you trust.
Reading the qualification probability
A supplier with a 98 percent lot acceptance rate has about an 82 percent chance of passing 10 consecutive lots, since 0.98 to the tenth power is about 0.82. At 95 percent the chance falls to about 60 percent, and at 90 percent to about 35 percent. This is why the rule works as a filter: only consistently good suppliers qualify. It also shows why a single rejection during qualification is not always a sign of a bad supplier, so look at the cause.
Common mistakes
Skip lot is not a way to avoid inspection on suppliers you simply trust. Typical errors are applying it to a new supplier or a changed process, forgetting to resume full inspection after a rejection or a major change, and applying it to critical characteristics that your contract requires on every lot. Document the rules, record the lot counter for each supplier and part, and review the status regularly.
- Exclude critical safety characteristics unless the customer agrees.
- Reset the counter after a rejection, a process change or a supplier relocation.
- Keep checking certificates and identification on skipped lots.
Frequently Asked Questions
How many lots must be accepted before skip lot is allowed?
Many procedures require about 10 consecutive accepted lots under normal inspection, and ISO 2859-3 sets qualification rules depending on the product and process. Your quality manual or the customer contract defines the number to use. The calculator lets you enter your own.
What happens when a skipped lot frequency lot is rejected?
In a typical procedure the supplier returns to lot by lot inspection and must requalify by passing the required number of consecutive lots again. Your procedure may also trigger a supplier corrective action request and a review of the process.
Is skip lot the same as reduced inspection in Z1.4?
No. Reduced inspection in Z1.4 uses a smaller sample on every lot. Skip lot inspects only some lots, but uses the normal sample size on the ones it does inspect. They can be combined in some procedures, but they are separate mechanisms.
Does skip lot work for new suppliers?
Not normally. Qualification depends on a documented history of accepted lots, a stable process and an approved quality system. New suppliers should begin with normal or even tightened inspection until they build a record.
If lot counters and supplier status are tracked in spreadsheets, we can discuss how to keep them in your ERP so the correct inspection level is applied automatically at receipt.
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