Bài giảng Operations Management - Supplement 10: Acceptance Sampling

Acceptance SamplingMcGraw-Hill/Irwin Copyright © 2012 by The McGraw-Hill Companies, Inc. All rights reserved.You should be able to:Explain the purpose of acceptance samplingContrast acceptance sampling and process controlCompare and contrast single- and multiple-sampling plansDetermine the average outgoing quality of inspected lots10S-*Student SlidesAcceptance samplingA form of inspection applied to lots or batches of items before or after a process, to judge conformance with predetermined standards May be applied to both attribute and variable inspection10S-*Student SlidesAcceptance sampling is most useful when at least one of the following conditions exists:A large number of items must be processed in a short timeThe cost consequences of passing defectives are lowDestruction testing is requiredFatigue or boredom caused by inspecting large numbers of items leads to inspection errorsStudent Slides10S-*Sampling plans:Plans that specify lot size, sample size, number of samples, and acceptance/rejection criteriaSingle-sampling planDouble-sampling planMultiple-sampling planStudent Slides10S-*Single-sampling planOne random sample is drawn from each lotEvery item in the sample is inspected and classified as “good” or “defective”If any sample contains more than a specified number of defectives, c, the lot is rejectedStudent Slides10S-*Double-Sampling PlanAllows the opportunity to take a second sample if the results of the initial sample are inconclusiveTwo values are specified for the number of defective itemsA lower level, c1An upper level, c2 If the number of defectives in the first sample is≤ c1 the lot is accepted and sampling is terminated> c2 the lot is rejected and sampling is terminatedBetween c1 and c2 a second sample is collectedThe number of defectives in both samples is compared to a third value, c3 If the combined number of defectives does not exceed this value, the lot is accepted; otherwise, it is rejectedStudent Slides10S-*Multiple-sampling planSimilar to a double-sampling plan except more than two samples may be requiredA sampling plan will specify each sample size and two limits for each sampleThe limit values increase with the number of samplesIf, for any sample, the cumulative number of defectives found exceeds the upper limit specified, the lot is rejectedIf for any sample the cumulative number of defectives found is less than or equal to the lower limit, the lot is accepted.If the number of defectives found is between the two limits, another sample is takenThe process continues until the lot is accepted or rejectedStudent Slides10S-*An important sampling plan characteristic is how it discriminates between high and low qualityOC curves describe a sampling plan’s ability to discriminateOC curveProbability curve that shows the probabilities of accepting lots with various fractions defectiveStudent Slides10S-*A typical OC Curve for ProportionsStudent Slides10S-*No sampling plan perfectly discriminates between good and bad qualityThe degree to which a sampling plan discriminates is a function of the graph’s OC curveSteeper OC curves are more discriminatingStudent Slides10S-*Acceptable quality Level (AQL)The percentage level of defects at which consumers are willing to accept lots as “good”Lot tolerance percent defective (LTPD)The upper limit on the percentage of defects that a consumer is willing to acceptStudent Slides10S-*Consumer’s risk, βThe probability that a lot containing defects exceeding LTPD will be acceptedManufacturer’s risk, αThe probability that a lot containing the acceptable quality level will be rejectedStudent Slides10S-*The AQL indicates good lots, and the LTPD indicates bad lotsStudent Slides10S-*An interesting feature of acceptance sampling is that the level of inspection automatically adjusts to the quality of the lots being inspected, assuming rejected lots are subjected to 100 percent inspectionGood lots have a high probability and bad lots a low probability of being accepted.If the lots inspected are mostly good, few will end up going through 100 percent inspection.The poorer the quality of the lots, the greater the number of lots that will come under close scrutiny10S-*Student Slides