Cpk vs Ppk: shortterm vs longterm process variation
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When calculating SPC control limits,
When calculating SPC control limits, many find out that even the data they use to calculate those limits already breaks rules within that same reference period. That shouldn't be possible, right? Standard deviation calculations tend to estimate wider variation than what's observed in the sample data.
Well, it is. And it's because of how sigma is estimated: not by calculating the standard deviation over the whole sample, but by estimating it based on the average (moving) range observed.
The same happens with Cpk vs Ppk - with Cpk using that same estimation from range and Ppk calculating over all values in the data set.
That is by design, though, because for SPC and Cpk you want to know short-term variation and specifically try to differentiate between everyday process variations and real process shifts.
#continuousimprovement #sixsigma #spc
Well, it is. And it's because of how sigma is estimated: not by calculating the standard deviation over the whole sample, but by estimating it based on the average (moving) range observed.
The same happens with Cpk vs Ppk - with Cpk using that same estimation from range and Ppk calculating over all values in the data set.
That is by design, though, because for SPC and Cpk you want to know short-term variation and specifically try to differentiate between everyday process variations and real process shifts.
#continuousimprovement #sixsigma #spc
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در تاریخ 1402/04/09 منتشر شده
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