Quality Control

ACCEPTANCE SAMPLING#Gaither Chapter 18#INTRODUCTIONQuality control is an activity in which measures are taken to control quality of future output.Sampling refers to observation of a population or lot for the purpose of obtaining some information about it.Acceptance sampling is a quality control technique.#WHAT IS SAMPLING?Sampling is the act, process, or technique of selecting a suitable sample, or a representative part of a population for the purpose of determining parameters or characteristics of the whole population.

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#DEFINITIONAcceptance sampling is defined as sampling inspection in which decisions are made to accept or reject products or services.

It is a decision making tool by which a conclusion is reached regarding the acceptability of lot.#advantageseliminates poor lots & improve overall quality of product.reduces inspection costs & risk.results may be more accurate.A rejected lot is frequently a signal to the manufacturer that the process should be improved.#8DESIGNING THE PLANAcceptable Quality Level (AQL) = Max. acceptable percentage of defectives defined by producer. (Producers risk) = The probability of rejecting a good lot.Limiting Quality Level (LQL) = Lot Tolerance Percent Defective (LTPD) = Percentage of defectives that defines consumers rejection point. (Consumers risk) =The probability of accepting a bad lot.#Typical Application of Acceptance SamplingThe decision to accept or reject the shipment is based on the following set standards:Lot size = NSample size = nAcceptance number = cDefective items = dIf d c, reject lot

#9Gaither Chapter 18#OC Curve CalculationTwo Ways of Calculating OC CurvesBinomial DistributionPoisson formula

Binomial DistributionCannot use because:Binomials are based on constant probabilities.N is not infinitep (fraction defect) changes

#10Vaughn(112-113)Gaither Chapter 18#OC Curve CalculationA Poisson formula can be used

Poisson is a limit Limitations of using Poissonn20 and p cd< cReplaceDefectivesn Nondefectivesd Defectives

Found in SampleReject LotAccept Lot#Gaither Chapter 18#DOUBLE SAMPLING PLANS Define:n1 -- sample size on first samplec1 -- acceptance number for first sampled1 -- defectives in first samplen2 -- sample size on second samplec2 -- acceptance number for both samplesd2 -- defectives in second sample

Take sample of size n1 Accept if d1 c1; reject if d1 > c2; Take second sample of size n2 if c1 < d1 c2Accept if d1+d2 c2; reject if d1+d2 > c2 #Conditions of Double-Sampling Plan:-One small sample is drawn initially.If the number of defectives is less than or equal to some lower limit, the lot is accepted.If the number of defectives is greater than some upper limit, the lot is rejected.If the number of defectives is neither, a second larger sample is drawn.Lot is either accepted or rejected on the basis of the information from both of the samples.

#Gaither Chapter 18#Double-Sampling PlanLot of N ItemsRandomSample ofn1 ItemsN n1 ItemsInspect n1 Itemsd1 > c2d1 < c1ReplaceDefectivesn1 Nondefectivesd1 Defectives

Found in SampleReject LotAccept LotContinuec1 < d1 < c2(to next slide)#Gaither Chapter 18#(d1+ d2) > c2Double-Sampling PlanN n1 ItemsRandomSample ofn2 ItemsN (n1 + n2)ItemsInspect n2 ItemsReplaceDefectivesn2 Nondefectivesd2 Defectives

Found in SampleReject LotAccept LotContinue(d1+ d2) < c2(from previous slide)#Gaither Chapter 18#Double sampling plansAdvantagesCan reduce the total amount of inspectionAllows the vendor a second chanceDisadvantagesCan lose the economical advantageMore record keeping is needed

#Multiple-sampling plan..(n,p)Acc the lotReject the lotSn1c1(n1+n2)c1