Conducting the "are all points within the control limits" test when there is no LCL?

T

tjkkj

#1
I created an R chart, and it had no LCL. I am conducting runs tests. How do I conduct the "are all points within the control limits" test when there is no LCL? There is one point that is very low on the graph that is concerning me.
 
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harry

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#2
Re: Need help with Runs Test

I created an R chart, and it had no LCL. I am conducting runs tests. How do I conduct the "are all points within the control limits" test when there is no LCL? There is one point that is very low on the graph that is concerning me.
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Steve Prevette

Deming Disciple
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#3
Re: Conducting the "are all points within the control limits" test when there is no L

Generally speaking, if you have data that cannot have negative numbers (counting certain things, time to complete something, Ranges in your example) and the LCL mathematically works out to be a negative number, then this means that the chart automatically passes for stability on that criteria.

Let's say you have an event that happens about once a month. Getting a month without the event is nothing special. That is what the negative LCL is telling you - getting a single zero is nothing special. Now, other rules such as 7 points in a row below average (this number varies with author) would allow you to detect a decreasing trend.
 
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