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  Statistical Techniques and 6 Sigma
  precontrol and process capability

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Author Topic:   precontrol and process capability
Lurker (<10 Posts)

Posts: 1
From:Danbury, CT
Registered: Jul 2001

posted 26 July 2001 02:10 PM     Click Here to See the Profile for dmac   Click Here to Email dmac     Edit/Delete Message   Reply w/Quote
Are there any statistical formulas that relate precontrol data to process capability indices?

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Posts: 53
From:Wellington, OH USA
Registered: Feb 2001

posted 26 July 2001 02:55 PM     Click Here to See the Profile for D.Scott   Click Here to Email D.Scott     Edit/Delete Message   Reply w/Quote
dmac - I am not sure this is what you are looking for but here it is anyway for what it is worth.

To establish a pre-control chart, it must be assumed that the process is normally distributed with a Cpk of at least 1.0. It can then be statistically shown that 86% of the product will be within the control lines with 7% in each of the other sections.

As there is no statistical data taken or analyzed, it is important to only use pre-control on a process known to be stable.

I am sure there are others who can add a lot to this but at least it may help to get you started.


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From:Tehran - IRAN
Registered: May 2001

posted 30 July 2001 12:53 PM     Click Here to See the Profile for MSAFAI   Click Here to Email MSAFAI     Edit/Delete Message   Reply w/Quote
1- Check normality by using a Histogram of your data.

2- You can have a ROUGH estimate of process capability by taking standard deviation of your data and calculating Ppk.

3- I don't know to what extent you can rely on the above calculation, because using a Pre-control chart you cannot be sure your process is under control (a prerequisit to determining capability)

Any other ideas?


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