# X-bar / S – chart sample size.

Six Sigma – iSixSigma Forums Old Forums General X-bar / S – chart sample size.

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• #29223

Devesh Dahale
Participant

I have a situation where end of shift data is summarized for three machines automatically and printed in the form of a report. Typically each machine runs about 800 to 1000 parts every shift. The mean and standard deviation of the parameter of interest of the parts run on each of the three machines during the shift is obtainable from the report. Mean and standard deviation is available for each machine. So, essentially I am trying to apply a control chart for a sample size of over 800 pieces (data poiints).
What value of “B4” should be used for calculating the control limits for the “S – chart”?
What value of “A3” should be used for calculating the control limits for the X-bar chart?Devesh Dahale

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#74206

Erik L
Participant

Devesh,
Why would you be interested in so many values?  I’m assuming, since you can calculate a meaningful mean and standard deviation that you have contiuous data.  So, if that’s the case why would you make the rational subgroup so high?  Values of constants that I’ve seen don’t typically go higher than a n=100.  If you do a COV study and show minimal variation within shift, why not simply pick a random sample of 10 or 20 measures from the shift to represent the output.  Typically this is the range of sub-groups where we shift to the Xbar-S chart.
Regards,
Erik

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#74210

Ashok Sarkar
Participant

If all the data is available in the system why we need X-bar S cahart. If it is possible to get the data then we may try different control chart like X-MR, CUSUM, EWMA  depending on what shift we are interested to monitor.
Otherwise we may sample the data for charting purpose!

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#74212

Marc Richardson
Participant

My question follows those that preceeded me. You have population data. You know what every part measures. Why do you need to use statistics? Statistics are used to infer population characteristics when all one has is sample data.
Marc Richardson
Sr. Quality Assurance Engineer

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#129290

Andhale
Participant

Is this Devesh Dahale of Pune, India…. If yes, hello.. this is Deepak Varma — mail me at [email protected] OR [email protected]
Phone : +919840730391
rgds
deepak

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