1. Lot by Lot A.S. attributes 2. Continuous production A.S. for attributes 3. A.S. for variables. LOT BY LOT ACC SAMP PLANS FOR ATTRIBUTES

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1 A.S. SYSTEMS 1. Lot by Lot A.S. attributes 2. Continuous production A.S. for attributes 3. A.S. for variables. LOT BY LOT ACC SAMP PLANS FOR ATTRIBUTES MIL-STD-105 E and ANSI/ASQ Z Initially developed for US govt. purchase i.e. Defence Ind. Developed in 1942 by Bell Tel. Labs Adopted by ISO ISO/DIS Applicable to (i) finished parts (ii) materials in process etc. For use in continuing series of lots Standard provides for Single Double Multiple Provision for NORMAL, TIGHTENED and REDUCED Inspection acceptancesampling shari.fkm 1

2 Comparison between OC for Tightened, Reduced and Normal Inspection Tightened when recent quality has deteriorated Reduced when recent quality is exceptionally good acceptancesampling shari.fkm 2

3 AQL MIL-105E uses AQL as basis for selecting plan; most impt. part of the standard. AQL max. % nc considered satisfactory as process average for A.S. purpose. AQL is designated in contract bet. consumer and producer AQL determined from historical data, judgment, engrg. info., producers capability, etc. Standard AQL range 0.010% 10% Non-conformity per unit per 100 units Usually use AQL 0.1 for critical n.c or major nc 2.5 for minor and c (acceptance no.) for critical should be zero (0) acceptancesampling shari.fkm 3

4 Usually start with NORMAL Acceptance criteria more stringent in TIGHTENED Switching allowed, say from NORMAL to tightened quality worse or NORMAL to reduced quality improves Function of actual qual. Nonconformities classification Critical nonconformity - can result in '( " ) hazardous or safety cond. prevent performance of func.! " #$ %& Major nonconformity result in failure, non-func. Minor nonconformity mostly appearance acceptancesampling shari.fkm 4

5 Sample size Determined by lot size and insp. Level 3 inspection levels I, II, III - same protection for producer - diff. protection for consumer Usually choose Level II Decision of sample size level a) level of protection b) function of type of prdt. Inexpensive Level II Prdt. Safety Level III In MIL-IO5E need to have AQL Sample size Code letter 4 additional special levels S1, S2, S3, S4 when small sample sizes needed with high sampling risks tolerated. acceptancesampling shari.fkm 5

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7 Implementation Steps MIL-STD-105D 1. Determine lot size (what is N) 2. Determine inspection level (usually II) 3. Find sample size Code Letter 4. Determine AQL 5. Determine type of sampling plan (SSP, DSP) 6. Find out the sampling plan using standard. 7. Start with normal change to tightened or reduced based on switching rules. acceptancesampling shari.fkm 7

8 Acceptance Sampling2 Single Sampling Plan Example : Lot size 2000 units AQL 0.65% Insp. Level II Find Single Sampling Plans for SSP N n Ac Re N N Ac Re a) normal b) tightened c) reduced Explanation N Ac Re 2000 acceptancesampling shari.fkm 8

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12 SWITCHING RULES N T When normal insp. being used switch to tightened when 2 out of 5 consecutive lots rejected eg. Case (I) Case (II) 1 st Lot 2 nd Lot 3 rd Lot 4 th Lot 5 th Lot OK NG NG OK OK NG OK OK NG OK T N When at tightened, change to normal after 5 consecutive lots accepted on original insp. N R 4 conditions 1. Preceding 10 lots OK 2. Total no. nc units limit number (see Table) 3. Production at steady state 4. when consumer considers desirable to reduce. acceptancesampling shari.fkm 12

13 Reduced to Normal 4 conditions 1. a lot or batch rejected 2. sampling plan results show neither accept or reject lot is accepted but return too normal 3. Production irregular or delayed 4. Others, eg. customer wants normal insp. to be instituted. acceptancesampling shari.fkm 13

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15 ALL OF THESE 1. Past 10 lots, all accepted. 2. Total nonconforming in last 10 lots Limit number (use table 10.5). 3. Prod steady 4. Consumer desire it ) SWITCHING RULES REDUCED ANYONE OF THESE 1. A lot not accepted 2. Neither accept/ reject accept & go back to normal. 3. Prod irregular or delayed. 4. Other conditions. START HERE NORMAL 5 Consecutive Lots accepted 2 out of 5 consec batches rejected. TIGHTEN acceptancesampling shari.fkm 15