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 Statistical Techniques and 6 Sigma
 Table of constant for Control Charts
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| Author | Topic: Table of constant for Control Charts |
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yii Lurker (<10 Posts) Posts: 4 |
posted 17 August 2001 12:59 AM
   
Does anyone have a table of constant for control chart for n > 25? IP: Logged |
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D.Scott Forum Contributor Posts: 53 |
posted 17 August 2001 07:46 AM
Table of constants n=25 A - 0.600 >25 - use 3/sqrt(n) for A & A2 Source"Quality Control and Industrial Statistics" (Acheson J Duncan) Hope this is what you need Dave [This message has been edited by D.Scott (edited 17 August 2001).] IP: Logged |
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yii Lurker (<10 Posts) Posts: 4 |
posted 19 August 2001 09:13 PM
What about d2, d3, D1-D4? Any method of calculation? IP: Logged |
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D.Scott Forum Contributor Posts: 53 |
posted 20 August 2001 06:55 AM
Sorry, but the source I have states "the fourth significant figures for (D) are in doubt for n greater than 5". It does not even show a calculation for sample size greater than 25. Maybe someone else can help shed some light. Dave IP: Logged |
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Rick Goodson Forum Wizard Posts: 135 |
posted 20 August 2001 09:38 AM
yii, Why do you want to take sample sizes greater than 25? Rick IP: Logged |
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yii Lurker (<10 Posts) Posts: 4 |
posted 20 August 2001 08:23 PM
One of the engineers requested to increase the sample size from 20 to 30 and he was asking for constants for n = 30. Now that pose an interesting question. Why the cutoff of n = 25? What's the explanation behind it? IP: Logged |
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Rick Goodson Forum Wizard Posts: 135 |
posted 21 August 2001 10:43 AM
yii, Without getting into the statistical proofs the reason involves the central limit theorm. If you recall Shewhart's experiments we find that if many samples of any sample size n are taken from a unoiverse, the averages (X-bar values) of the samples will form a frequency distribution and the average of the averages (X-Double Bar) of that frequency distribution will tend to be near the average of the universe (u, mu). The spread of the X-bar values of the frequency distribution will depend on the spread of the universe and the sample size (n) with the spread of the X-bar values being smaller as n gets larger. In the long run the standard deviation of the X-bar values will be the standard deviation of the population divided by the square root of the sample size. This holds regardless of the shape of the universe. Now with that in mind, the subgroup size for a control chart is basically an economical decision. We chose a 'rational subgroup' so that the variation within the units is small. If the variation with in a subgroup represents the piece-to-piece variability over a short period of time, then unusual variation between subgroups would reflect changes in the process that should be investigated. The practical problem we face is whether to take large samples less frequently or smaller samples more frequently. The longer we wait between samples the longer the process may run in an out of control condition if an assignable cause enters the picture. Keep in mind also that as the sample size increases the control limits get smaller moving toward the central line of the chart. If you want more information on the statistics involved try these texts: Quality Control and Industrial Statistics by Acheson Duncan Regards, Rick IP: Logged |
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