BS ISO 39511:2018 pdf download

BS ISO 39511:2018 pdf download.Sequential sampling plans for inspection by variables for percent nonconforming (known standard deviation)
1 Scope
This International Standard specifies sequential sampling plans and procedures for inspection by variables of discrete items. The plans are indexed in terms of producer’s risk point and the consumer’s risk point. Therefore, they are suitable not only for the purposes of acceptance sampling, but for the more general purpose of the testing of simple statistical hypotheses for proportions. The purpose of this International Standard is to provide procedures for the sequential assessment of inspection results that may be used to induce the supplier to supply lots of a quality having a high probability of acceptance. At the same time, the consumer is protected by a prescribed upper limit to the probability of accepting a lot (or process) of poor quality.
This International Standard is primarily designed for use under the following conditions:
a) where the inspection procedure is to be applied to a continuing series of lots of discrete products all supplied by one producer using one production process. In such a case, sampling of particular lots is equivalent to the sampling of the process. If there are different producers or production processes, this International Standard shall be applied to each one separately;
b) where only a single quality characteristic x of these products is taken into consideration, which must be measurable on a continuous scale;
c) where the measurement error is negligible (i.e. with a standard deviation no more than 10 % of the process standard deviation);
d) where production is stable (under statistical control) and the quality characteristic x has a known standard deviation, and is distributed according to a normal distribution or a close approximation to the normal distribution;
In this International Standard, it is assumed that, where double specification limits apply, conformance to both specification limits is either equally important to the integrity of the product or is considered separately for both specification limits. In the first case, it is appropriate to control the combined percentage of product outside the two specification limits. This is referred to as combined control. In the second case, nonconformity beyond each of the limits is controlled separately, and this is referred to as separate control.
5 Principles of sequential sampling plans for inspection by variables
Under a sequential sampling plan by variables, sample items are drawn at random and inspected one by one, and the cumulative leeway (which measures a “distance” between the process level and specification limits) is obtained. After the inspection of each item, the cumulative leeway is compared with the acceptability criteria in order to assess whether there is sufficient information to determine lot or process acceptability at that stage of the inspection. If, at a given stage, the cumulative leeway is such that the risk of accepting a lot of unsatisfactory quality level is sufficiently low, the lot is considered acceptable and the inspection is terminated. If, on the other hand, the cumulative leeway is such that the risk of non-acceptance of a lot of satisfactory quality level is sufficiently low, the lot is considered not acceptable and the inspection is terminated. If the cumulative leeway does not allow either of the above decisions to be taken, then an additional item is sampled and inspected. The process is continued until sufficient sample information has been accumulated to warrant a decision that the lot is acceptable or not acceptable.