Ch 5 Methods for SPC - INDE321QualityControl Instructor:...

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INDE 321 Quality Control Instructor : Linda Boyle University of Washington Dept. of Industrial & Systems Engineering Dept. of Civil & Environmental Engineering Chapter 5 Methods and Philosophy of SPC
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Chapter 5 Statistical Process Control (SPC) Collection of problem-solving tools Used to reduce process variability Used to stabilize your process
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Magnificent Seven Seven major tools for basic SPC problem- solving: Histogram or Stem and Leaf plot Check Sheet Pareto Chart Cause and Effect Diagram Defect Concentration Diagram Scatter Diagram Control Chart
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Causes of Quality Variation Chance causes e.g., background noise in statistical control . Assignable causes e.g., operator errors, defective parts, improperly adjusted machine out of control . Goal of SPC is to reduce variability due to assignable causes.
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Control Charts A typical control chart (or Shewhart control chart) Has control limits set at calculated values If process is in control, nearly all points will lie between upper control limit (UCL) and lower control limit (LCL).
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Basic Control Chart Upper Control Limit Lower Control Limit Center Line In  control
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Control Charts Detecting Out-of Control Situations If at least one point plots beyond the control limits. If all the points are within the control limit, but behave in a systematic or nonrandom manner.
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Control Charts Relationship to hypothesis testing Example : A process has a true process mean, μ = 74 a process standard deviation, σ = 0.01 Samples of size, n=5 are taken. The sample standard deviation of the sample average, , is 0045 . 0 5 01 . 0 n x = = σ = σ x
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From our last lecture (parameter estimates): ( 29 ( 29 ( 29 1 1 1 1 2 1 2 1 2 2 2 1 2 1 2 2 2 1 2 1 2 1 n n n n X V X V X X V X X X X σ = = + = + = + = -
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Control Charts Relationship to hypothesis testing Control limits can be set at 3 standard deviations from the mean. This results in “3-Sigma Control Limits” UCL = 74 + 3(0.0045) = 74.0135 CL = 74 LCL = 74 - 3(0.0045) = 73.9865 x σ 0027 . 0 where 2 = α Z
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Control Charts Relationship to hypothesis testing Choosing the control limits is equivalent to setting up the critical region for testing hypothesis H 0 : μ = 74 H 1 : μ 74
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Control Charts Relationship between the process and the  control chart One  measurement Sample
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Control Charts Things to remember Most processes do not operate in a state of statistical control Routine and attentive use of control charts will identify assignable causes . Control charts only detect assignable causes. Management, operators, and engineering action are needed to eliminate assignable causes.
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Control Charts Two general types of control charts Variables Control Charts Apply to data that follows a continuous distribution (e.g., measurement data).
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This note was uploaded on 04/26/2010 for the course INDE 321 taught by Professor Boyle during the Winter '10 term at University of Washington.

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Ch 5 Methods for SPC - INDE321QualityControl Instructor:...

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