# Spread for X-Double Bar and R-Bar Chart?

Six Sigma – iSixSigma Forums General Forums New to Lean Six Sigma Spread for X-Double Bar and R-Bar Chart?

Viewing 4 posts - 1 through 4 (of 4 total)
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• #55130

leaning
Participant

Hello! Still studying, but have this question I am stuck on. The key says that answer C is the correct one, but all I can find is that is the formula for the UCL for the Xbar chart.

Can that one formula be used to calculate spread??

Regards,

Leaning

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#198810

Mike Carnell
Participant

@learning69 Rbar/d2 is a way to estimate the std, dev. using the mean. C is just X double bar plus 3 estimated standard deviations. Is that the spread they are talking about? No clue. I would think if you wanted to discuss the process spread then it should +/-.

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#198811

Craig Tickel
Participant

I agree with Mike, the formula that contains A2 X R-bar is really 3 R-bar/D2 divided by the Sqrt(n) to account for the sample size. Since they are asking for the spread of the individaul values, not the distribution of samples, should not be included in the formula, hence the 3 R-bar/d2… but as he stated should be +/-…

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#198816

Chris Seider
Participant

I find it a poorly worded question. If they had stated, where would you expect 99.73% of the data for the INDIVIDUALS to be, then C is clearly correct.

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