Question

Calculation of Capability Indices

  • 31 January 2022
  • 2 replies
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C
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Need your help in understanding the capability indices calculation.

As per IFS, to calculte such quality indices, at least one Control Plan must exist, and at least two Analyses for a Control Plan line in this Control Plan must be in status “complete”. I totally agree with the first prerequisite but do not understand the second one.

In fact, in our  company, we are manufaturing small run of parts and some of them can only be manufactured once a year. And so, we still must calculate the Cp/Cpk/mean on one shop order and cannot wait the next year to have a second shop order on the same part to estimate the proportion of parts that are within specification limits.

Am I mistaken or can IFS fulfiil our requirements in another way ?

Thanks for your help

 

 

 

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2 replies

Peter Lundgren
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The recommended practice in many industries is to calculate capability only after a process has been demonstrated to be in a state of statistical control. The data should be collected long enough, and in such way, as to include all common sources of variation, preferably for 25 or more inspections.

Calculating Cp and Cpk after only one, small run, shop order is far from an ideal situation from a statistical point of view. If you really want to do it this anyway you can create two Analysis (inspections) for your one and only shop order. Use a control plan line percentage Inspection Code that gives you a sample size of 50% of the lot size. Create two Analysis for the shop order operation of interest. Inspect the first 50% items of the lot size on the first Analysis, and the remaining 50% of the lot size on the second Analysis.

C
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@Peter Lundgren 

Thanks a lot for your feedback and I agree with you on the process.

What you suggest seems to meet our requirement.

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