Bài giảng Operations Management - Supplement 10 Acceptance Sampling

Acceptance SamplingSupplement 10Copyright © 2015 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.You should be able to:LO 10S.1 Explain the purpose of acceptance samplingLO 10S.2 Compare and contrast single- and multiple-sampling plansLO 10S.3 Describe what an operating characteristic curve doesLO 10S.4 Determine the average outgoing quality of inspected lotsSupplement 10: Learning ObjectivesAcceptance 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 inspectionAcceptance SamplingLO 10S.1Purpose of Acceptance SamplingThe purpose of acceptance sampling is to decide whether a lot satisfies predetermined standardsLots that satisfy these standards are passed or acceptedRejected lots may be subjected to 100 percent inspectionIn the case of purchased goods, they may be returned for credit or replacementLO 10S.1Single-Sampling PlanSingle-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 rejectedLO 10S.2Double-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 rejectedDouble-Sampling PlanLO 10S.2Multiple-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 rejectedMultiple-Sampling PlanLO 10S.2Operating Characteristic (OC) CurveAn 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 defectiveLO 10S.3OC CurveA typical OC Curve for ProportionsLO 10S.3Acceptable Defect LevelsAcceptable 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 acceptLO 10S.3An OC CurveThe AQL indicates good lots, and the LTPD indicates bad lotsLO 10S.3An 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 scrutinyAverage Quality of Inspected LotsLO 10S.4Average outgoing qualityAverage of rejected lots (100 percent inspection) and accepted lots (a sample of items inspected)Average Outgoing QualityLO 10S.4In practice, the last term of the AOQ formula is close to 1.0Eliminate this term, soConstruct the AOQ curve for this situationN = 500, n = 10, and c = 1AOQp.05.10.15.20.25.30.35.40pac.9193.7361.5443.3758.2440.0860.0860.0464AOQ.046.074.082.075.061.045.030.019LO 10S.4AOQ CurveApproximate AOQ = .082LO 10S.4