# Running runchart for Discrete measure

Six Sigma – iSixSigma Forums Old Forums General Running runchart for Discrete measure

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• #51983

RK
Member

Hi,
My metric is – any item beyond 90 days is a defect. It is a discrete measure and target is DPMO reduction. How to I use a run chart here to check the stability? I have data on a weekly basis.
thanks for the help in advance
RK

0
#182164

Remi
Participant

Hai RK,
didn’t you learn that a continuous metric is preferable to a categorical metric? And now you take a ‘continuous’ metric and change it into a categorical one. Just one advise: DON’T!!!!
Use the metric “# of days” with Upper Spec = 90. This makes life much easier. Be a little careful with some calculations because this metric is not normal distributed. If you improve (=lower) Mean and Sigma of this metric your dpmo will go down.
Tip: weekly 1 measurement will make your analysis, improve and control phase a long term endeavour. Just try to get the measurement per product. It is there somewhere otherwise the weekly summary could not be made.
Remi

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#182308

RK
Member

remi,
Thanks for the reply…..
I know for sure that continuous metric is anyday better than discrete…but that is the customer requirement that we take up discrete….so can’t help it
But You still hav’nt answered my question…..

RK

0
#182355

Remi
Participant

Hai RK,
sorry.
1> Use the categorical CTQ as an externalCTQ and the continuous one as an internalCTQ. Work with the internal one (Measure, Analyse, Improve) and show end reults (to bobo’s) on external one.
2> Use p-chart to show time behaviour of your categorical CTQ. p-chart is for %. 1 DPMO is just the same as 0.000100 % DPO. The definition is different but the statistical usage is the same.
Remi

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#182377

Helper
Participant

Hello RK,
You do have a couple of options depending on how your data is collected.
1.  In stead of using a run chart you can use an XMR (Imr) chart; your CTQ can be items during the collection cycle that go beyond 90 days.
2.  You can also use a proportion of defectives chart.
Both of these types can acomodate discrete data for your purpose and let you know if the process is stable.
Hope this helps…..

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