Quality: Acceptance Sampling

Single sampling plan is characterized by

N = lot size n = sample size c =acceptance number If c or less non-conforming units are found in the sample, the lot is accepted, otherwise it is rejected.

Multiple sampling plan

is similar to the double sampling plan in that successive trials are made, each of which has acceptance, rejection and inconclusive options.

Three approaches to "lot sentencing"

1.Accept with no inspection 2.100% inspection 3.Acceptance Sampling

Three important aspects of sampling

1.Involves random sampling of the entire lot 2.Accept and Reject Lots (does not improve the quality) "Lot Sentencing" 3.May be used as an audit Tool

Double sampling

allows to take a second sample if the results of the original sample are inconclusive.

Sampling plans

specify the lot size, sample size, number of samples and acceptance/rejection criteria

A single sampling plan is one where

•A representative sample of n items is drawn from a lot size of N items •Each item in the sample is examined and classified as good/defective •If the number of defective exceeds a specified rejection number (c) the whole lot is rejected; otherwise the whole lot is accepted

First sample possible decisions

•If the quality is good, nonconforming units ≤ c1, accept lot •If the quality is not good, nonconforming units ≥ r1, reject lot •If between c1 and r1, take a second sample

Acceptance sampling is useful when:

•Large numbers of items must be processed in a short amount of time. •The cost of passing nonconforming product is relatively low. •Fatigue/boredom is caused by inspecting large numbers of items •Destructive testing is required. •Cost of 100% inspection is high in relation to cost of passing a nonconforming unit. •When performance of producer's quality using SPC is not available. •When automatic inspection is not available.

Disadvantages of acceptance sampling

•Risk of accepting "bad" lots and rejecting "good" lots •Requires planning and documentation •Less information generated, but is usually enough •No assurance the entire lot conforms or doesn't conform to specs.

Sampling plans can be

•Single sampling plans •Double sampling plans •Multiple sampling plans -Sequential sampling plans

Random Sampling

•Units selected for inspection should be chosen at random •Random sampling means that each unit in the lot has the same opportunity to sampled •If random samples are not used, bias can be introduced which would likely increase risks •If judgement methods are used to select the sample, the statistical basis of the acceptance-sampling procedure is lost Stratification consideration

Second sample possible decisions

•accepted if the total nonconformities ≤ c2 or •rejected if the total nonconformities ≥ r2