Lesson plan
process performance index
In process improvement efforts, the process performance index is an estimate of the process capability of a process during its initial set-up, before it has been brought into a state of statistical control. Formally, if the upper and lower specifications of the process are USL and LSL, the estimated mean of the process is \hat{\mu} , and the estimated variability of the process (expressed as a standard deviation) is \hat{\sigma} , then the process performance index is defined as: : \hat{P}_{pk} = \min \Bigg[ {USL - \hat{\mu} \over 3 \hat{\sigma}}, { \hat{\mu} - LSL \over 3 \hat{\sigma}} \Bigg] \hat{\sigma} is estimated using the sample standard deviation. P pk may be negative if the process mean falls outside the specification limits (because the process is producing a large proportion of defective output). Some specifications may only be one sided (for example, strength). For specifications that only have a lower limit, \hat{P}_{p,lower} = {\hat{\mu} - LSL \over 3 \hat{\sigma}} ; for those that only have an upper limit, \hat{P}_{p,upper} = {USL - \hat{\mu} \over 3 \hat{\sigma}} .…
Article summary
In process improvement efforts, the process performance index is an estimate of the process capability of a process during its initial set-up, before it has been brought into a state of statistical control. Formally, if the upper and lower specifications of the process are USL and LSL, the estimated mean of the process is \hat{\mu} , and the estimated variability of the process (expressed as a standard deviation) is \hat{\sigma} , then the process performance index is defined as: : \hat{P}_{pk} = \min \Bigg[ {USL - \hat{\mu} \over 3 \hat{\sigma}}, { \hat{\mu} - LSL \over 3 \hat{\sigma}} \Bigg] \hat{\sigma} is estimated using the sample standard deviation. P pk may be negative if the process mean falls outside the specification limits (because the process is producing a large proportion of defective output). Some specifications may only be one sided (for example, strength). For specifications that only have a lower limit, \hat{P}_{p,lower} = {\hat{\mu} - LSL \over 3 \hat{\sigma}} ; for those that only have an upper limit, \hat{P}_{p,upper} = {USL - \hat{\mu} \over 3 \hat{\sigma}} .…
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
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- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
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