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IES 331 Quality Control

IES 331 Quality Control . Chapter 14 Acceptance Sampling for Attributes – Single Sampling Plan and Military Standard Week 13 August 30 – September 1, 2005. Acceptance Sampling. Another area of quality control and improvement Closely connected with inspection and testing of product

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IES 331 Quality Control

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  1. IES 331 Quality Control Chapter 14 Acceptance Sampling for Attributes – Single Sampling Plan and Military Standard Week 13 August 30 – September 1, 2005

  2. Acceptance Sampling • Another area of quality control and improvement • Closely connected with inspection and testing of product • Inspection can occur at many points in a process Acceptance Sampling: the inspection and classification of a sample of nits selected at random from a larger batch or lot and ultimate decision about disposition of the lot – Lot DispositionorLot Sentencing Two common pointsof inspection

  3. Acceptance Sampling

  4. 3 approaches for sentencing 1. _________________ • For low cost or low impact material on the subsequent process • From trusted supplier 3. ___________________ • when 100% inspection is too costly • Trusted supplier with potential risk 2. ____________________ • Inspect every item in the lot, then remove the defective units by discarding, reworking, or returning to suppliers • For critical process. Defective input may result high failure cost • From new supplier

  5. Advantages Less expensive because of less inspection ___________________ ___________________ Rejection on entire lot motivates quality improvement for suppliers Disadvantages Risk of _______________ Risk of _______________ Requires planning and documentation Require extensive study on customer’s requirement Advantages and Disadvantages of Sampling

  6. Types of Sampling Plan • Single-sampling plan • Double-sampling plan • Multiple-sampling plan • Sequential sampling plan Sampling Plan for Variables Sampling Plan for Attributes ___________________________ _____________________________

  7. Lot Formation • Lots should be ______________ • produced on the same machines, by same operators, from common raw materials, at approximately the same time period • Larger lots are better than smaller lots • Lots should be conformable to the material handling systems • ___________________________________

  8. Random Sampling • Inspected units should be selected at random • Inspected units should represent all items in the lot • Potential bad story ____________________ __________________ __________________

  9. Single-Sampling Plans for Attributes • A lot of size N has been submitted for inspection • Sample size n • Acceptance number of defective c • Lot sentencing is based on one sample of sizen Example N = 10,000 n= 89 c = 2 N = 10,000 n = 89

  10. Single-Sampling Plans for AttributesThe OC Curve • Operating characteristic (OC) curve • Also refer to Chapter 2; Page 78 Curve plots the probability of acceptingthe lot(Pa) versus the lot fraction defective (p)

  11. Single-Sampling Plans for AttributesThe OC Curve

  12. Theoretically , it can be achieved by ___________________ Due to sampling, ideal OC curve cannot be perfectly achieved. If sample size (n) _______, the OC curve shape will approach the ideal OC curve Single-Sampling Plans for AttributesIdeal OC curve

  13. Single-Sampling Plans for AttributesEffects of non OC curves • As sample size (n) __________, the OC curve shape will approach the ideal OC curve • Note that c is kept in proportional to n • ____________________

  14. Single-Sampling Plans for AttributesEffects of con OC curves • Changing acceptance number (c) does not significantly change the shape • __________________________________________

  15. Single-Sampling Plans for AttributesOC curve Acceptable quality level (AQL) • The poorest quality level for the supplier’s process that a consumer would consider to be acceptable • A property of the supplier’s manufacturing process, not a property of the sampling plan Lot tolerance percent defective (LTPD) • The protection obtained for individual lots of poor quality • Also called rejectable quality level (RQL) and the limiting quality level (LQL) • LTPD is a level of lot quality specified _____________, not a characteristic of the sampling plan • Sampling plans can be designed to have specified performance at the AQL and the LTPD points ______________________________________ ______________________________________

  16. Single-Sampling Plans for AttributesType-A and Type-B OC Curves

  17. Designing a single-sampling plan with a specified OC curve To construct sampling plan such that Probability of acceptance for lots with fraction defective p1 Probability of acceptance for lots with fraction defective p2

  18. Binomial Nomograph

  19. Single-Sampling Plans for AttributesRectifying Inspection • When the lot is rejected, it requires corrective action

  20. Average outgoing quality - AOQ Given the lot size of N, sample size of n, and fraction defective of p • n items in the sample that, after inspection, contain no defectives, because all discovered defectives are replaced • N – n items that, if the lot is rejected, also contain no defectives • N – n items that, if the lot is accepted p(N-n) defectives If N >> n, then

  21. Average outgoing quality - AOQ

  22. Military Standard 105E • Sampling procedure for inspection by attributes developed during World War II and is the most widely used acceptance-sampling system for attributes in the world today • A collection of sampling schemes; therefore an acceptance-sampling system • Provides for three types of sampling: single, double, and multiple • Primary focal point is the acceptable quality level (AQL) • Different AQLs may be designated for different types of defects: critical, major, and minor • Generally specified in contract or by authority responsible for sampling • Sample size is determined by lot size and by choice of inspection level

  23. Military Standard 105E

  24. Military Standard 105E Procedure • Choose the AQL • Choose the inspection level • Determine the lot size • Fine the appropriate sample size code letter (from Table 14-4) • Determine the appropriate type of sampling plan to use (single) • Enter the appropriate table to find the type of plan to be used • Determine the corresponding normal and reduced inspection plans to be used when required

  25. Discussion of Military Standard • Several points about MIL STD 105E should be emphasized: • MIL STD 105E is AQL-oriented • Not all possible sample sizes are possible (2,3,5,8,13,20,32,50, etc.) • Sample sizes are related to lot sizes • Switching rules are subject to criticism for both misswitching between inspection plans and discontinuation even though there has been no actual quality deterioration

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