Control Charts - ControlCharts PurposeofControlCharts

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    Control Charts
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    Purpose of Control Charts A graph of an attribute over time Provides the average and variability of a quality  characteristic over time Used to judge quality over time and spot  trends Can spot potential problems before they get bad Can determine whether the process is “in control”,  working properly (normal variability)
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    General Format  A plot of the average of samples taken  over time with: Upper control limits Lower control limits Overall (population) mean
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    Types of Control Charts Variables charts For wts., sugar, color, texture, etc. X or mean charts Means of several samples over time R or range charts Range (variability) of several samples over time Usually for production line samples, but  also finished goods for raw materials
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    Types of Control Charts Attributes charts When classifying as defective or ok Raw products and finished goods
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    Preliminary Considerations What is to be measured? Wts., sugar, color, etc. How measured (accuracy, etc.)? Where measured (on the line, in lab)? When measured (regular time  intervals)? How many samples at each time  (usually 5)?
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    Variables Charts When processing line is working  normally: Select 5 samples at regular time intervals  (50-100 total samples) Determine means and ranges for each  group of 5 samples at each time interval Determine overall mean and range Calculate upper and lower control limits
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    Variables Charts Upper and lower control limits for mean Upper control limit for range Lower control limit for range A 2 , D 4 , D 3  from Table R A X LCL UCL 2 , ± = R D UCL 4 = R D LCL 3 =
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    Variables Charts UCL and LCL are 3 standard deviation  units above/below the mean
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This note was uploaded on 03/03/2011 for the course FOS 4722 taught by Professor Sims during the Spring '10 term at University of Florida.

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Control Charts - ControlCharts PurposeofControlCharts

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