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Deciding on when to go for Cp & Cpk or Pp & Ppk Study?

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statdoug

#3
Re: How to decide , when to go for Cp ,Cpk study or for Pp,Ppk Study?

Hi,
The difference between the Cp indices and the Pp indices is the method in which the standard deviation is calculated. The Pp indices use the standard deviation from all of the data where the Cp indices use the pooled "instantanious" standard deviation from the subgroup ranges or standard deviations.
If a process is unstable, the standard deviation in the Pp indices will be inflated. Generally (if the range/sigma chart is in control), the standard deviation used in the Cp indices will not be inflated by process instability.
one way to look at these is that the Cp indices will give you an indication of what the process will produce if stable, the Pp indices "indicate" what it is actually producing. This should be taken with a grain of salt however, as different aspects of instability can over-inflate the standard deviation. You really can not estimate the process capability on an unstable process.

As an interesting side note, Shewhart proposed a ratio of these two estimates of standard deviation as one of his tests for instability (his test 2).

Doug
 

bobdoering

Stop X-bar/R Madness!!
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#4
Re: How to decide , when to go for Cp ,Cpk study or for Pp,Ppk Study?

...You really can not estimate the process capability on an unstable process.
Nor can you utilize these specific indices on a stable process that is not bilateral and from a normal distribution.

For all practical purposes, they should be considered estimates, anyway. Using one number to represent the behavior of a process over time is nearly as valid as using a winning lottery ticket to represent the value of all lottery tickets.
 
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