# Seven sun rule help

Six Sigma – iSixSigma Forums Old Forums General Seven sun rule help

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• #53037

Tierradentro
Participant

This is a question question about why seven is used in the seven run rule for seven continuous controls points below or above the mean. I have researched a lot of QA databases and I haven’t been able to get an answer to this question. Any assistance will be greatly appreciated.

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#187561

Mikel
Member

Perhaps it has something to do with the likelihood of that event

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#187601

Schafheutle
Participant

Hi John
the reason is very straitforward: How big is the probability that the next point in a data series is below or above the previous one? 0.5. How big is it that 3 in a row are all above or below the all previous? 0.5 x 0.5 = 0.25. So having 7 in a row all above or below the previous makes 0.5×0.5×0.5×0.5×0.5×0.5=0.015. Five in a row have the probability of 0,063. Depending on the risk of drawing wrong conclusions you would prefer 5 or 7 in a row. The risk will vary between 1.5 to 6.3%. Or otherwise said the probability of being right is 94.7 to 98.5%.
Are you familiar with these numbers? They are in the standard range of 2 to 3 sigma which is the control charting agreement of having noise or special causes. So having 5 to 7 datapoint steadily increasing or decreasing point with high probability to a special cause effect.
Regards Markus

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#187616

anupam
Participant

EXCELLENT EXPLAINATION !!

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#187622

Owen Chang
Participant

Thanks a lot! It really a good lesson learned for me.
Happy New Year!

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#187632

SPC rulez
Member

Masterbook of SPC, answers for all questions (original language of course ;-))
http://www.teknologiainfo.net/default.asp?docId=12360&productId=14367

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#187633

SPC rulez
Member

Broken link, another try:
http://www.teknologiainfo.net/default.asp?docId=12360&productId=14367&fromSearch=true
Or by google: “Suorituskykyiset prosessit, hyödynnä SPC”

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