11Disadvantages of Inspection WastefulSampling and inspection add cost and decrease valueInaccurateEven 100% inspection is only 80% effective because of the possibility of human errorsImpractical(Costly)Inspection may involve destructive testing
12Disadvantages of Inspection Wrong messageInspection communicates to people and suppliers that bad parts will still be tolerated.RisksIn sampling and inspection there is a risk of accepting bad lots and rejecting good lotsNo continuous improvementSampling is still inspection, not prevention, so that quality is not typically continuously improved.
13Advantages of a Stable Process Management and workers know the process capability and can predict performance, costs and quality levels.Productivity will be at a maximum and costs will be minimized.Management will be able to measure the effects of changes in the system with greater speed and reliability.
14Advantages of a Stable Process If management wants to alter specification limits, it will have the data to back up its decision.(A stable process does not necessarily meet specs nor exhibit minimal variation - it’s just predictable)
15Acceptance SamplingAcceptance sampling has three basic decisions: accept, reject, or resample.Reason for using acceptance sampling:Cost of passing defects is lowDestructive testing is requiredCost of inspection high relative to cost of lossAssumes stable processLarge number of items must be processed in a short time
16Acceptance Sampling Terms Producer’s Risk (): Risk of rejecting a lot with acceptable quality level. (type I error)Consumer’s Risk (): Risk of accepting a lot with unacceptable quality level. (type II error)Acceptable Quality Level (AQL): The maximum percentage defective that can be considered satisfactory.Lot Tolerance Percent Defective (LTPD): The percent defective where the consumer desires the probability of acceptance to be at a low level.
17Acceptance Sampling - Attributes Types of plansSingleN, n, c(1000, 50, 1)DoubleN, n1, n2, c1, c2 , c3(3000, 50, 80, 1, 3, 5)Sequentialn, ca, cr(50, 0, 4); (50, 1, 5)
19Acceptance Sampling - Variables AdvantagesSmaller sample than equivalent attribute planProvides more informationProvides insight into quality improvementsDisadvantagesSeparate plan for each variableInspection costs are higherDistribution estimate required
20Acceptance Sampling - Variables Process ParameterAverage quality of the product/process or variability of the quality is knownSingle Specificationn & XaDouble Specificationn, XLa, XUaLot Proportion NonconformingForm 1 (k-method)Form 2 (M-method)
22Advantages & Disadvantages of Attribute Charts Some quality characteristics can only be viewed as a attribute.Quality characteristic may be measurable as a variable but an attribute is used for time, cost or convenience.Combination of variables can be measured as an attribute rather than use a multivariate chart.
23Advantages & Disadvantages of Attribute Charts Attributes don’t measure the degree to which specifications are met or not met.Doesn’t provide much information on cause.Variable chart can indicate potential changes which allow preventive actions.Larger sample size required.
24Types of Attribute Charts p-Chart - Fraction NonconformingCan have constant or variable sample size.Good tool for relating information about average quality level.np-Chart - Number of NonconformingNumber of nonconforming items may be easier for user to understand.
25Types of Attribute Charts c-Chart - Number of NonconformitiesUsed when desire is to control the number of defects where one defect may not cause the entire product to be defective.Often used where area of opportunity is continuous and a constant size
26Types of Attribute Charts u-Chart - Number of Nonconformities/unitArea of opportunity is of variable size.U-Chart - Number of Demerits/unitAllows the use of variable weights for different classes of defects.