Process Stability

hi everyone, i have some questions i would like to ask about Process Stability in Statistical Process Control.

1.Can i know how do we check whether our process is stable or not using SPC?.
2. Can i say that by looking at a variable control chart and say if there is no Out of Control (OOC), my process is stable at the moment?.
3. Can i say that if my Cpk and Ppk for this month is almost equivalent to each other, generally my process is stable?.


Sincerely,
Hazwan..

hazwan2283 said:

hi everyone, i have some questions i would like to ask about Process Stability in Statistical Process Control.

1.Can i know how do we check whether our process is stable or not using SPC?.

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The lack of any out of control signals (and usually there are more than just the outside the control limits rule) would indicate you are stable.

hazwan2283 said:

2. Can i say that by looking at a variable control chart and say if there is no Out of Control (OOC), my process is stable at the moment?.

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I'd say it has been stable since the beginning of the chart if there are no OOC points (again, from a balanced set of rules)

hazwan2283 said:

3. Can i say that if my Cpk and Ppk for this month is almost equivalent to each other, generally my process is stable?.

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No. You need a longer history than just this month, and best to use the SPC rules.

hazwan2283 said:

Sincerely,
Hazwan..

Click to expand...

3. In addition to Steve’s response (which was perfectly correct) you need to be very careful about making assumptions regarding stability from Cpk and Ppk. You can have a non homogenous process that is perfectly stable and your Cpk will be much smaller than your Ppk. This the reason for Rational Subgrouping.