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  Statistical Techniques and 6 Sigma
  Statistical Process Control

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Author Topic:   Statistical Process Control
Sankaran Mahadevan
Lurker (<10 Posts)

Posts: 4
From:Jamshedpur, Jharkand, India
Registered: Aug 2001

posted 21 August 2001 11:17 PM     Click Here to See the Profile for Sankaran Mahadevan   Click Here to Email Sankaran Mahadevan     Edit/Delete Message   Reply w/Quote
Hi guys,

The variation in any Statistical Process Control includes also the measuring equipment variation. If the Process is in Statistical Control and Process Capability index is fairly high say at 2, then is it necessary to do Measurement System Analysis (Gage R &R study). The Control Chart is maintained for all the three shifts and three operators are using the same chart. So in this case I think that Appraiser variation is also taken into account. Kindly comment on this point.

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Posts: 16
From:Northbrook, IL, USA
Registered: Jul 2001

posted 22 August 2001 11:21 AM     Click Here to See the Profile for KenK     Edit/Delete Message   Reply w/Quote
Given your scenario, I would venture the following statements:

1. If the observed process variation is considered sufficient, then there probably is not necessarily a real need to know the contribution of the measurement system to that variation. Certainly it is not bad enough to make the process variation appear unacceptable.

2. Regardless, you will still need to run studies investigating bias, linearity, and stability, since those are location inaccuracies rather than variation issues. You could think you're OK when you're really not.

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Al Dyer
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Posts: 814
From:Lapeer, MI USA
Registered: Oct 2000

posted 22 August 2001 11:48 AM     Click Here to See the Profile for Al Dyer   Click Here to Email Al Dyer     Edit/Delete Message   Reply w/Quote
I guess my only comment would be he following:

1- Was a gage R&R done prior to production with three different operaters.

2- Gage R&R is based on using 3 random people to measure the effectiveness of the gage.

3- Gage stability and linearity are based on the gage, not the person using the gage.

4- Always consider the use of the gage. Are gage blocks used as a gage or are calipers verified by a calibrated gage blocks.

Just a personal view and I have been wrong in the past


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Posts: 42
From:Princeton, IN, USA
Registered: Feb 2001

posted 22 August 2001 04:03 PM     Click Here to See the Profile for AJLenarz   Click Here to Email AJLenarz     Edit/Delete Message   Reply w/Quote
Sankaran –

The real question here is…. what are you obligated to? I can’t speak on behalf of the TS world, but if you operate in the QS realm, you must perform “appropriate statistical studies… to analyze the variation present in the results of each type of measuring and test equipment system.”

These studies may consist of bias, linearity, stability, repeatability and reproducibility studies.

The key word here is “appropriate”. Choose the appropriate statistical studies relevant to your measuring system and process and be prepared to explain your choice.

I would tend to concur. With a Cpk of 2, I would question the necessity of a GR&R. The exception might be in a situation with high Cpk requirements.

But i'm just a pup in the world of quality.

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