09/16/04SJSU Bus David Bentley1 Chapter 10S – Acceptance Sampling Definition, purpose, sampling plans, operating characteristic curve, AQL, LTPD, risks

09/16/04SJSU Bus David Bentley2 Acceptance Sampling Form of inspection applied to lots or batches Before or after a process, not during the process Purpose: determine whether lot satisfies predetermined standards

09/16/04SJSU Bus David Bentley3 Sampling Plans Plan defines: Lot size Sample size Number of samples Acceptance/rejection criteria

09/16/04SJSU Bus David Bentley4 Single-sampling Plans One random sample from each lot Every item in sample inspected and classified: good or bad If number of defects (bad) > specified limit, lot is rejected

09/16/04SJSU Bus David Bentley5 Double-sampling Plans Upper and lower defect limits set for each lot Results of initial sample Defects ≤ lower limit: lot accepted Defects ≥ higher limit: lot rejected Defects between two limits: take 2 nd sample If 2 nd sample needed, compare number of defects from both samples to third value If number of defects (bad) > specified value, lot is rejected

09/16/04SJSU Bus David Bentley6 Multiple-sampling Plans More than 2 samples 2 limits specified for each sample Cumulative number of defects compared to limits for each successive sample Defects ≤ lower limit: lot accepted Defects ≥ higher limit: lot rejected Defects between two limits: continue sampling

09/16/04SJSU Bus David Bentley7 Operating characteristic curve Stevenson, William J., Operations Management, 8 th Edition (mod. 09/15/04 DAB) Shows probabilities of accepting lots with various defect fractions Probability of accepting lot Lot quality (fraction defective) 3%

09/16/04SJSU Bus David Bentley8 Acceptance Levels Acceptable Quality Level (AQL) Customers may be willing to accept lots containing less than a specified % defects Lot Tolerance Percent Defective (LTPD) Upper limit of defects tolerated in accepted lots

09/16/04SJSU Bus David Bentley9 Type I and Type II Errors Type I (producer’s risk) Conclusion: non-randomness is present and the process is out of control Action: stop the process which was OK Reality: randomness is present and the process is in control Type II (consumer’s risk) Conclusion: randomness is present and the process is in control Action: don’t stop the process; continue making defects Reality: non-randomness is present and the process is out of control