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TRAINING SESSION

TRAINING SESSION. ACCEPTANCE SAMPLING CONFIDENCE INTERVALS. THESE BOXES CONTAIN 1000 PERFECT SNAP CAPS EACH. THE V.P. FOR CORPORATE QUALITY APPROVED A $250,000 BUDGET TO INSPECT, X-RAY, MRI, ETC. EACH SNAP CAP IN THE BOX TO INSURE THAT EACH ONE IS ACCEPTABLE TO USE IN PRODUCTION

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TRAINING SESSION

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  1. TRAINING SESSION ACCEPTANCE SAMPLING CONFIDENCE INTERVALS

  2. THESE BOXES CONTAIN 1000 PERFECT SNAP CAPS EACH • THE V.P. FOR CORPORATE QUALITY APPROVED A $250,000 BUDGET TO INSPECT, X-RAY, MRI, ETC. EACH SNAP CAP IN THE BOX TO • INSURE THAT EACH ONE IS ACCEPTABLE TO USE IN PRODUCTION • I.E. GUARANTEED 100% PERFECT QUALITY

  3. BOX WITH 1000 PERFECT SNAP CAPS • WHO WANTS TO MAKE THE DECISION TO RELEASE THESE 1000 SNAP CAPS FOR USE IN PRODUCTION????? • EASY DECISION!!!!! • BOX CONTAINS 0% DEFECTIVE

  4. NOW! WHAT IF? • EXACTLY ONE SNAP CAP IS DEFECTIVE • NOW, WHO WANTS TO MAKE THE DECISION TO RELEASE THE LOT, CONTAINING “ONE” DEFECTIVE SNAP CAP, FOR USE IN PRODUCTION????? • ONE DEFECTIVE OUT OF 1000 IS WHAT WE WOULD REFER TO AS “.1% DEFECTIVE”

  5. THE REAL DECISION! • ARE YOU WILLING TO LIVE WITH THIS QUALITY LEVEL (.1% DEFECTIVE)? • IF YES, WHAT ABOUT? • TWO DEFECTIVES OUT OF 1000 OR .2% DEFECTIVE

  6. WHAT ABOUT? • 3 DEFECTIVES OUT OF 1000 OR .3% DEFECTIVE • 4 DEFECTIVES OUT OF 1000 OR .4% DEFECTIVE • ……… • WHAT ABOUT 500 DEFECTIVES OUT OF 1000 OR 50% DEFECTIVE

  7. REALITY • WHO WANTS TO PLAY THE SHELL GAME? • “A” BOX SHOWS UP AND YOU MUST MAKE THE DECISION WHETHER OR NOT TO RELEASE THE BOX TO PRODUCTION • WHAT IS YOUR DECISION?????? • HOW DO YOU DECIDE??????

  8. HOW DO WE KNOW WHAT THE LOT % DEFECTIVE IS?

  9. Definition of the terms AQL and RQL: • AQL = ACCEPTABLE QUALITY LEVEL (THE MAXIMUM % DEFECTIVE YOU CONSIDER ACCEPTABLE) • RQL = REJECTABLE QUALITY LEVEL (THE MINIMUM % DEFECTIVE YOU CONSIDER UNACCEPTABLE)

  10. MANAGEMENT DECISION

  11. ANOTHER ISSUE! • YOU’RE USING A SAMPLING PLAN WITH A 1.5% AQL (SHIPPERS) • THE SAMPLE SIZE IS N=32 AND YOU ACCEPT ON 1 OR FEWER DEFECTIVES • YOU CONDUCT THE TEST, FIND 1 DEFECTIVE, AND ACCEPT THE LOT • 1 DEFECTIVE IN A SAMPLE OF 32 IS 3.1%, NOT 1.5% (AQL) • HOW CAN THIS BE?

  12. EXAMPLE: SNAP CAPSThe Way You Normally Do It! • YOU JUST RECEIVED A “LOT” OF SNAP CAPS FROM YOUR SUPPLIER • DO YOU WANT TO ACCEPT THE LOT AND RELEASE IT TO PRODUCTION? • HOW DO YOU DECIDE IF THE LOT IS ACCEPTABLE? • DEFINE “ACCEPTABLE”

  13. EXAMPLE: SNAP CAPSThe Way You Normally Do It! • ACCEPTABLE CAN BE DEFINED MANY DIFFERENT WAYS • VARIABLES (DIMENSIONS) VERSUS ATTRIBUTES (GOOD/BAD) • FOR ATTRIBUTES, WE NORMALLY TALK ABOUT THE % DEFECTIVE • “ACCEPTABLE” = MAX % DEFECTIVE

  14. EXAMPLE: SNAP CAPSThe Way You Normally Do It! • REFERENCE: ANSI Z1.4 • INSPECTION BY ATTRIBUTES • LOT SIZES = 35,001-150,000 • SAMPLE SIZE CODE LETTER “N” • SINGLE, NORMAL, LEVEL 2 • AQL = 1.5% • SAMPLE SIZE = N = 500 • ACCEPT IF # DEFECTIVES < 14 (C = 14)

  15. SINGLE ATTRIBUTE ACCEPTANCE SAMPLING PLAN • CHARACTERIZED BY A SAMPLE SIZE (n) AND ACCEPT NUMBER (C) SUCH THAT IF YOU HAVE C OR FEWER DEFECTIVES IN THE SAMPLE OF SIZE n, YOU WILL ACCEPT THE LOT. OTHERWISE, YOU REJECT THE LOT.

  16. OTHER ACCEPTANCE SAMPLING PLANS • DOUBLE SAMPLING • MULTIPLE SAMPLING • SEQUENTIAL SAMPLING • CONDITIONAL SAMPLING • ETC.

  17. SNAP CAPS • IS N = 500, C = 14 A GOOD ACCEPTANCE SAMPLING PLAN?????? • HOW DO YOU DECIDE?????? • WELL, IT DEPENDS!!!!!!

  18. ASSOCIATED WITH EVERY ACCEPTANCE SAMPLING PLAN IS AN OPERATING CHARACTERISTIC CURVE • THE “OC” CURVE FOR • SAMPLE SIZE = 500 • ACCEPT LOT IF # DEFECTIVES < 14 • REJECT LOT IF # DEFECTIVES > 15 • CAN BE SHOWN TO BE:

  19. ROLE FOR ACCEPTANCE SAMPLING PLANS • ACT AS A GATE KEEPER! • IF INCOMING QUALITY (% DEFECTIVE) IS WHAT YOU DEFINE AS ACCEPTABLE (in this case 1.5% defective or less), • YOU WANT THE GATE TO STAY OPEN AND ACCEPT ALL (MOST) OF THE LOTS.

  20. WHAT HAPPENS? • WHEN A LOT IS ACCEPTED AND LATER EVENTS INDICATE A PROBLEM (OPEN CAPS) • AFTER 100% INSPECTION, YOU FOUND 3000 DEFECTIVE CAPS IN THE LOT OF 100,000 SNAP CAPS!!!!!!! • (3000/100,000) = 3% • WHY DID YOU ACCEPT THE LOT????

  21. WE NEED TO MAKE A MANAGERIAL DECISION! • DO YOU WANT TO RELEASE THIS LOT TO PRODUCTION? • WELL, IT DEPENDS! • DO YOU CONSIDER A LOT WITH 3% DEFECTIVES • JUST UNDESIRABLE, OR • DEFINITELY UNACCEPTABLE

  22. NEW ROLE FOR ACCEPTANCE SAMPLING PLANS • IF INCOMING QUALITY (% DEFECTIVE) IS WHAT YOU DEFINE AS UNACCEPTABLE (let’s say 3% in this case, RQL = 3%) • YOU WANT THE GATE TO STAY CLOSED AND REJECT ALL (MOST) OF THE LOTS.

  23. THIS IS THE PRIMARY ROLE ACCEPTANCE SAMPLING PLAYS!

  24. REALITY OF AQL AND RQL • IF WE CHOOSE • AQL = 1.5% • RQL = 3% • WHAT DOES THAT REALLY MEAN?

  25. AQL = 1.5% : WHAT DOES IT MEAN • LOT SIZE = 100,000 SNAP CAPS • 98,500 ARE GOOD • 1,500 ARE BAD • LOT IS ACCEPTABLE!!!! • LOT QUALITY: (1500/100,000)= 1.5%

  26. BY DEFINITION • THIS LOT IS ACCEPTABLE TO USE IN PRODUCTION ???(1.5% ARE DEFECTIVE) • QUESTION? AS A MANAGER, WOULD YOU REALLY RECOMMEND THAT PRODUCTION USE THIS LOT TO PRODUCE PRODUCT AND SHIP TO THE CUSTOMER? “YES OR NO” • IF “NO” THEN WE NEED TO START OVER.

  27. RQL = 3% : WHAT DOES IT MEAN • LOT SIZE = 100,000 SNAP CAPS • 97,000 ARE GOOD • 3,000 ARE BAD • LOT IS UNACCEPTABLE • LOT QUALITY: (3000/100,000 = 3%)

  28. BY DEFINITION • THIS LOT IS UNACCEPTABLE TO USE IN PRODUCTION (3% ARE DEFECTIVE) • YOU WOULD NOT WANT TO ACCEPT THIS LOT!!!!!!

  29. MANAGERIAL CONCLUSION • A LOT OF 100,000 SNAP CAPS CONTAINING 1500 OPEN CAPS IS ACCEPTABLE (1.5%) • HOWEVER, • A LOT OF 100,000 SNAP CAPS CONTAINING 3000 OPEN CAPS IS UNACCEPTABLE (3%)

  30. IS (n = 500, C = 14) A GOOD SAMPLING PLAN? • IF OUR QUALITY IS AT THE AQL (1.5%) WE ACCEPT MOST LOTS • HOWEVER, IF OUR QUALITY IS AT THE RQL (3%) WE STILL ACCEPT 45% OF THE LOTS

  31. CONCLUSION • NOT A GOOD SAMPLING PLAN! • WHAT SHOULD WE DO?????? • INCREASE THE SAMPLE SIZE!! • TENDENCY IS TO USE Z1.4 AND SELECT A LARGER SAMPLE SIZE FOR THE SAME AQL = 1.5% • N = 800, C = 21 • PROBLEM SOLVED, OR IS IT?????

  32. WHAT DID WE GET FOR OUR MONEY? • IS A REDUCTION (FROM 45% TO 32%)IN THE PROBABILITY OF ACCEPTING LOTS WITH 3% DEFECTIVES • WORTH A 60% INCREASE IN SAMPLE SIZE (FROM 500 TO 800) • HOW DO WE REDUCE THIS PROBALILITY OF ACCEPTING LOTS WITH 3% DEFECTIVE EVEN LOWER?

  33. HOW TO FIND RIGHT SAMPLING PLAN

  34. COMMUNICATION IS THE KEY TO UNDERSTANDING

  35. WHAT HAPPENS IF? • YOU FIX THE AQL AT 1.5% • AND VARY THE SAMPLE SIZE AND ACCEPT/REJECT NUMBER

  36. WHAT HAPPENS IF? • YOU FIX THE SAMPLE SIZE • AND VARY THE ACCEPT/REJECT NUMBER

  37. WRAP-UP THOUGHTS!! • ONLY YOU CAN DECIDE WHAT QUALITY LEVELS YOU’RE WILLING TO LIVE WITH. • ONLY YOU CAN DECIDE WHAT QUALITY LEVELS YOU’RE NOT WILLING TO LIVE WITH. • ONLY YOU CAN DECIDE IF THE COST OF ACCEPTANCE SAMPLING IS THE WISEST WAY TO SPEND YOUR LIMITED QUALITY ASSURANCE DOLLARS!

  38. WHAT ARE YOUR QUESTIONS ABOUT? • SPECIFIC SAMPLING PLANS YOU ARE CURRENTLY USING • EFFECT OF CHANGING SAMPLE SIZE FOR YOUR PLAN • COMPARISON BETWEEN TWO DIFFERENT PLANS • ???

  39. CONFIDENCE INTERVALS • A TOOL USED TO ESTIMATE (GUESS) WHAT YOU THINK THE TRUE PERCENT DEFECTIVE IS IN A LOT (PROCESS).

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