Sample sizes are designated by sample size code letters. Table 1 shall be used to find the applicable code letter for the particular lot size and the prescribed inspection level. NOTE For economy of space in the tables or to avoid unnecessary repetition in the text, the abbreviated term "code letter" is sometimes used.
The AQL and the sample size code letter shall be used to obtain the sampling plan from Tables 2, 3, 4 or For a specified AQL and a given lot size, the same combination of AQL and sample size code letter shall be used to obtain the sampling plan from the table for normal, tightened and reduced inspection.
When no sampling plan is available for a given combination of AQL and sample size code letter, the tables direct the user to a different letter. The sample size to be used is given by the new sample size code letter, not by the original letter.
If this procedure leads to different sample sizes for different classes of nonconformities or nonconforming items, the sample size code letter corresponding to the largest sample size derived may be used for all classes of nonconformities or nonconforming items, when designated or approved by the responsible authority.
As an alternative to a single sampling plan with an acceptance number of 0, the plan with an acceptance number of 1 with its correspondingly larger sample size for a designated AQL where available may be used, when designated or approved by the responsible authority. As another alternative, the optional fractional acceptance number plans described in clause 13 may be used when approved by the responsible authority.
Three types of sampling plans, single, double and multiple, are given in Tables 2, 3 and 4, respectively. When several types of plans are available for a given AQL and sample size code letter, any one may be used. A decision as to the type of plan, either single, double or multiple, when available for a given AQL and sample size code letter, shall usually be based upon the comparison between the administrative difficulty and the average sample sizes of the available plans.
For the sampling plans given in this part of ISO , the average sample size of multiple plans is less than for double, and both of these are less than the single sample size see Table 9.
Usually, the administrative difficulty for single sampling and the cost per item in the sample are less than for double or multiple sampling. To determine acceptability of a lot under percent nonconforming inspection, the applicable sampling plan shall be used in accordance with The number of sample items inspected shall be equal to the sample size given by the plan.
If the number of nonconforming items found in the sample is equal to or less than the acceptance number, the lot shall be considered acceptable. If the number of nonconforming items is equal to or greater than the rejection number, the lot shall be considered not acceptable.
The number of sample items first inspected shall be equal to the first sample size given by the plan. If the number of nonconforming items found in the first sample is equal to or less than the first acceptance number, the lot shall be considered acceptable.
If the number of nonconforming items found in the first sample is equal to or greater than the first rejection number, the lot shall be considered not acceptable. If the number of nonconforming items found in the first sample is between the first acceptance and rejection numbers, a second sample of the size given by the plan shall be inspected.
The number of nonconforming items found in the first and second samples shall be accumulated. If the cumulative number of nonconforming items is equal to or less than the second acceptance number, the lot shall be considered acceptable. If the cumulative number of nonconforming items is equal to or greater than the second rejection number, the lot shall be considered not acceptable.
In multiple sampling, the procedure shall be similar to that specified in In this part of ISO , there are five stages so that a decision will be reached by the fifth stage at the latest.
In order to determine the acceptability of a lot in a nonconformities per hundred items inspection, the procedure specified for nonconforming inspection see The operating characteristic curves for normal and tightened inspection, shown in Table 10, indicate the percentage of lots which may be expected to be accepted under the various sampling plans for a given process quality.
The curves shown are for single sampling, integer acceptance number plans; curves for double and multiple sampling are matched as closely as practicable. The OC curves shown for AQLs greater than 10 are applicable for inspection for number of nonconformities; those for AQLs of 10 or less are applicable for inspection for nonconforming items.
For AQLs of 10 or less these OC curves are also applicable to inspection for number of nonconformities. For each of the curves shown, values of the quality of submitted product corresponding to selected values of probabilities of acceptance are shown in tabular form.
In addition, values corresponding to tightened inspection, and values corresponding to sampling for number of nonconformities for AQLs of 10 or fewer nonconformities per items are also given. Normalized scheme OC curves found in Table 12 indicate the long-range percentage of lots of various qualities that will be accepted, taking into account the switching rules but disregarding the effect of the rule for discontinuation of inspection 9. The abscissa is the ratio of the process quality to the AQL.
Each curve represents an acceptance number for normal inspection. The process average can be estimated by the average percent nonconforming or average number of nonconformities per items whichever is applicable found in the samples of product submitted by the supplier for original inspection, provided that inspection was not curtailed.
When double or multiple sampling is used, only first sample results shall be included in the process average estimation. The AOQL is the maximum of the average outgoing qualities for all possible qualities submitted for a given acceptance sampling plan. Approximate AOQL values are given in Table 8-A for each of the single sampling plans for normal inspection and in Table 8-B for each of the single sampling plans for tightened inspection.
Average sample size curves for double and multiple sampling, as compared with the corresponding single sampling plan for each acceptance number, are given in Table 9.
These curves show the average sample sizes which may be expected to occur under the various sampling plans for given levels of process quality. The curves assume that the inspection is not curtailed see ISO , 2. This part of ISO is intended to be used as a system employing tightened, normal and reduced inspection on a successive series of lots to achieve consumer protection while assuring the producer that acceptance will occur most of the time if quality is better than the AQL. Occasionally, specific individual plans are selected from this part of ISO and used without the switching rules.
For example, a purchaser may be using the plans for verification purposes only. This is not the intended application of the system given in this part of ISO and its use in this way shall not be referred to as "inspection in compliance with ISO ". When used in this way, this part of ISO simply represents a repository for a collection of individual plans indexed by AQL. The operating characteristic curves and other measures of a plan so chosen shall be assessed individually for a plan from the tables provided.
If the series of lots is not long enough to allow the switching rules to be applied, it may be desirable to limit the selection of sampling plans to those, associated with a designated AQL value, that give consumer's risk quality not more than a specified limiting quality protection.
Sampling plans for this purpose can be selected by choosing a consumer's risk quality CRQ and a consumer's risk probability of lot acceptance to be associated with it. Table 6 applies when inspecting for nonconforming items and Table 7 applies when inspecting for number of nonconformities. When there is reason for protecting against a specified limiting quality in a lot, Tables 6 and 7 may be useful for fixing minimum sample sizes to be associated with the AQL and inspection level specified for inspection of the series of lots.
ISO gives details of the procedure for selecting sampling plans for lots in isolation. Table 6-A indicates that the minimum sample size shall be given by sample size code letter L. Tables 5-A, 5-B and 5-C give the probability of rejection for lots of AQL quality on normal, tightened and reduced inspections, respectively.
This probability is denoted as producer's risk in 2. This subclause specifies optional procedures for fractional acceptance number sampling plans. The optional procedures may be used with the approval of the responsible authority. Unless otherwise specified, standard procedures shown above shall be followed. Fractional acceptance number plans are found in Tables A, B and C. The use of fractional acceptance number plans does not require a change in sample size code letters, with the corresponding change in sample size, when the combination of sample size code letter and AQL results in a plan between the 0 and 1 acceptance number as described in When the fractional acceptance number sampling plans remain constant for all lots, the following rules apply.
Otherwise the current lot shall be considered not acceptable. If the first lot inspected has one nonconforming item, that lot is not accepted. If the given acceptance number is 1 or more, add 7 to the acceptance score. When, for fractional acceptance number plans, the updated acceptance score prior to inspection is 9 or more, the lot can be considered acceptable only if there is at most one nonconforming item in the sample.
When the acceptance number is an integer, use this acceptance number to determine acceptability in accordance with 1 1. The acceptance score shall be updated added to after obtaining the sampling plan but before deciding on the acceptability of the lot. The acceptance score shall be reset after the acceptability decision is made. In contrast, the switching score see 9.
NOTE When an acceptance score is used for the case of constant sampling plans, the results are the same as 1 3. In order to determine the acceptability of a lot when inspecting for number of nonconformities, the procedures specified for inspection for nonconforming items see These rules are the same as indicated in 9. The rule for updating the switching score 9. The rules are the same as indicated in 9.
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