Ch 6S SPC 2009 - Ch 6 Quality Control What is Statistical...

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Ch 6 Quality Control What is Statistical Process Control (SPC)? 1. Statistical process control is a statistical technique used to set standards, make measurements, and take corrective action as a product or service is being produced. What’s the objective of SPC? Provide a statistical signal when assignable cause of variations are present 1. What are the types of variation defined by Shewhart? 0. variations 0. Variation inherent in a process 1. Also named as random variation 1. variations 2. Variation due to specific causes 3. Can be modified through operator or management action 2. Types of data 2. data - Product characteristic evaluated with a discrete choice 0. Good/bad, yes/no 3. data - Product characteristic that can be measured 1. Length, size, weight, height, time, velocity 1
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3. Control charts 0. Graph establishing process control limits 1. What are the purposes of a control chart? 3. show changes in data pattern 4. A Process is in control if: 4. No sample points outside control limit 5. Most points near process average 6. About equal number of points above & below center line 7. Points appear randomly distributed 2 Control Charts R Chart Variables Charts Attributes Charts X Chart P Chart C Chart Continuous Numerical Data Categorical or Discrete Numerical Data Control Chart Types Control Chart Types
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f8e5 X Chart 0. i nterval or ratio scaled numerical data 1. Shows sample mean over time 2. Monitors process average 3. Example: Weigh samples of coffee & compute means of samples; .. In order for the X-Bar Chart to be meaningful, we must assume the variation in the process is constant (i.e. stable). Factors for Computing Control Chart Limits Sample size, n Mean Factor, A 2 Upper Range, D 4 Lower Range, D 3 2 1.88 3.268 0 3 1.023 2.574 0 4 .729 2.282 0 5 .577 2.115 0 6 .483 2.004 0 7 .419 1.924 0.076 8 .373 1.864 0.136 9 .337 1.816 0.184 10 .308 1.777 0.223 12 .266 1.716 0.284 3 X Chart Control Limits Sampl Range Tim # Sampl Mean Time i R A x x LCL R A x x UCL 2 - = 2 + = n R R i n 1 i = = n x i n i x 1 = =
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Example. Consider the problem faced by Janet Lugg, director of quality control measures. Her Plant produces frames for desktop computers which must meet certain size specifications. To ensure these standards are met, Janet collects k=24 samples (subgroups), each of size 6, and measures their width. The results are reported below. From these data, construct X-bar chart. Does the X-bar chart reveal any problem in the process?. C:\Documents and 
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This note was uploaded on 10/16/2009 for the course BUSQOM 1070 taught by Professor Shang during the Fall '09 term at Pittsburgh.

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Ch 6S SPC 2009 - Ch 6 Quality Control What is Statistical...

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