QM Knowledge: The Skip-Lot Procedure

Anyone involved in quality management and QM systems has probably already come across the term skip lot procedure. This article explains what this sampling procedure is all about and why it can be useful.

The term skip-lot (skip a lot) actually brings us pretty close to the general idea: Because this procedure is about skipping a certain subset of a series when checking. The procedure is a dynamic one, which always adapts to the individual circumstances. If the results of the previous tests were consistently OK and met certain criteria, a part of the series can either not be tested at all or be tested with significantly less effort in a new test. Clear advantage: By (partially) dispensing with tests, the overall testing effort is significantly reduced.

The simplified procedure benefits the customer in particular thanks to the large savings potential, even if he runs the risk of using defective products in his company. According to the applicable law, however, he must always carry out so-called identity and quantity checks in the incoming goods inspection despite the skip lot, i.e. check whether the correct goods have been delivered and whether the quantity is also correct, in order to have a claim against the supplier in case of doubt.­

How and to what extent the acceptor checks is usually agreed between the business partners. A look at the standard can provide a clue: Here, the skip-lot method is anchored in DIN ISO 2859-3 “Acceptance sampling based on the number of nonconforming units or defects (attribute testing) – Part 3: Skip-lot method”. This prescribes the so-called AQL method (Acceptable Quality Level), which is used to determine sample sizes or also to determine the acceptable quality limit. The predefined quantity of goods (sample) from the delivery lot must then be tested according to specified criteria. However, since this procedure allows high inaccuracies and is therefore not compatible with the requirement for “zero defect quality”, it is used less and less today.

An inspection using the skip-lot sampling method proceeds as follows: Seven testing levels can be dynamically passed, with the highest being 100 percent testing and the lowest (skip lot) being zero percent testing. The rules as to which dynamic level is applied are predefined and are formed from the results of the previous test in each case. Later in the company’s practice, the test results are further observed and corrected if necessary, so that the test sequence is continuously adjusted. Thus, if it turns out that defective parts were present in the skipped quantity after all, the following partial quantity is inspected again in a more elaborate manner. It is also possible that the affected – defective – partial quantity must be checked subsequently. In this case, the first-in-first-out principle must be strictly adhered to at goods receipt in order to be able to clearly identify the quantity concerned.

Applied consistently, the skip-lot procedure can significantly reduce the inspection effort with a manageable risk, save resources and thus support the quality management officer. However, the remaining risks should be known, because unnoticed, incorrectly introduced products can sometimes result in considerable follow-up costs.