Application of Decision Tables in Quality Management

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Sampling Plans

“Test Method Standard establishes three sets of matched sampling plans for the sampling inspection of a product submitted for acceptance. These sampling plans provide for inspecting the samples from lots or batches by attributes or variables measurement and for continuous sampling by attributes measurement. The three sets of matched sampling plans are indexed by seven specified verification levels (VL) and five code letters (CL), which are determined by the lot or production interval size. The sampling plans are matched between corresponding VL and CL combinations to result in essentially similar protection.” [1]
The parameters that are needed to determine the sampling plan are:

a. Verification level (VL): Identifies the level of significance of a characteristic for the user. The amount of effort to assure conformance can be allocated on the basis of importance to the user. VL-VII requires the highest level of effort, and the effort decreases as the VL decreases to the lowest level, VL-I. The VL's are specified in the contract or product specifications. Contractors are expected to produce and submit a product in full conformance to all requirements. Lots, batches, or production intervals of a product that consistently meet or exceed all requirements will be accepted by the sampling plans of this standard and will result in qualifying for reduced sampling levels.[2]
b. Type of sampling (attributes, variables, or continuous): The sampling type is attribute, if the characteristic controlled has some predefined states like ‘Ok, Not ok; Damaged, Not damaged’. If the characteristic controlled has some numerical values on a continuous scale like length, weight, then the type of sampling is variable. If the characteristic has to be controlled for a specific time period continuously, then the type of sampling is continuous. Throughout the applications, it is assumed that the sampling type is attribute.
c. Lot or production interval size code letter (CL): Code letter is determined by Verification Level and Lot Size. The table below is used. The code letters are specified by the letters from A to E.


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d. Switching procedure (normal, tightened, and reduced): The quality history of the items which are inspected has an influence on the kind of sampling plan that will be applied. There are three kinds of sampling plans, which are normal, tightened and reduced inspections. Tightened inspection is applied for the cases in which the last lots received were not accepted or the quality level of last lots was not satisfactory. Reduced inspection is used when the last lots inspected were all accepted and the quality level of the supplier is assumed to be reliable. In the non-extreme cases, normal inspection is used.
For a specific item that is received from a specific supplier for the first time, normal sampling plan is applied. In other words, either the part or the contractor is new, normal sampling plan is applied.
The type of inspection that should be used for the lot received is not a stationary variable, but it should be decided for each lot received by checking the quality history of the item. As a result of this, it is possible to switch between the sampling types. There are special procedures listed in the test method standard to switch from one inspection type to another.
According to the specifications in the standard, it is possible to switch between reduced and normal inspection and between tightened and normal inspection. As a result of this, the inspection type that will be used can either be the same inspection type as before or normal inspection.


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The procedures to switch from one inspection to another one include some conditions which are listed below.

“Normal inspection to tightened inspection: When normal inspection is in effect, tightened inspection shall be instituted when one of the following conditions occurs:
    • 2 lots have been withheld from acceptance within the last 5 or fewer lots.
Tightened inspection to normal inspection: When tightened inspection is in effect, normal inspection may be instituted when the following conditions are both satisfied:
    • The cause for producing the non-conformance is corrected.
    • 5 consecutive lots are accepted.

Normal inspection to reduced inspection: When normal inspection is in effect, reduced inspection may be constituted when the following conditions are all satisfied:
    • 10 consecutive lots are accepted while on normal inspection.
    • Production is at a steady rate.
    • The contractor’s quality system is considered satisfactory by the Government.
    • Reduced inspection is considered desirable by the Government.
Reduced inspection to normal inspection: When reduced inspection is in effect, normal inspection shall be instituted when one of the following conditions occurs:
    • A lot is withheld from acceptance.
    • Production becomes irregular or delayed.
    • The contractor’s quality system is unsatisfactory.
    • Other conditions warrant that normal inspection be re-instituted.”[4]

After deciding the code letter and inspection type to be used, in the next step it is needed to identify the sample size that will be inspected from the lot. Sample size is determined based on a table. If the inspection type is normal, then the normal sampling part of the table should be used.

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If the inspection type is decided to be reduced, then the sample size should be shifted one column to right in the sampling table. In other words, the reduced sampling part of the table should be used.
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If the inspection type decided is tightened inspection, then the sample size should be shifted one column to left in the sampling table.

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If the lot size is less than or equal to the sample size, 100% attributes inspection is required.
After deciding on the sample size, the samples are inspected. If there is even one nonconforming unit in the samples, the lot should be rejected.[6]
The flowchart below summarizes the steps of using the Test Method Standard.


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Find out more about the implementation of decision tables on test method standard >
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