# Control Chart with Range Calculation

Six Sigma – iSixSigma Forums General Forums New to Lean Six Sigma Control Chart with Range Calculation

Viewing 14 posts - 1 through 14 (of 14 total)
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• #55730

John Peters
Participant

I’m new to Lean Six Sigma and have been given a question (as seen below) and would really appreciate any help available.

Question: When using a control chart what would an average of 95.75 show in terms of the range when there was a sample of 20.
a) 1.0
b) 0.8
c) 0.5
d) 0.6

Any help would be appreciated and an explanation on how it was worked out would be amazing!

John

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#201396

Katie Barry
Participant

@johnpeters123 You’re more likely to get helpful responses if you first explain what you think/why.

The more details you provide, the better. The iSixSigma audience is helpful, but they like to see that someone is putting forth a good-faith effort first.

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#201397

John Peters
Participant

Hi Katie,

Yes, sorry i do understand that. But I have no idea how it could be answered with the information provided
.
When it states range I would assume that it would refer to the span of the data for the upper and lower control limits. However this just states that the ‘average’ is 95.75 with a sample of 20… so i’m completely unsure how you would go about workout out the range just from these figures.

Thanks anyway!
John

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#201398

Katie Barry
Participant

@johnpeters123 What you just wrote is exactly what I meant! :)

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#201399

John Peters
Participant

Hi Katie,

Thank you, so what you’re saying is it can’t be answered based on the information provided?

Thanks again!

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#201400

Chris Seider
Participant

it looks like homework here but….

think of the following. just because you know the center of your data, why would you presume you can describe the variation?

look at the formulas for your consideration.

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#201401

John Peters
Participant

Wow.

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#201402

Katie Barry
Participant

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#201403

John Peters
Participant

Sorry, accidentally hit send. Thanks for your help.

I’ll look into it.
No it’s not homework, it’s just revision for my green belt exam.

Thanks

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#201404

Robert Butler
Participant

If there is a way to answer that question I will have to admit I can’t think of a way to do it. You are correct – the usual problem of this type is to give a min and a max, compute their average and then divide the result by d2 for 20 samples (d2 comes from tables of percentage points of the distribution of the relative range) For 20 samples that would be 3.735.

I would say that if taken at face value – 95.75 is the mean and 20 is the sample size then there is no way to estimate the standard deviation and even if you did so the statement/question “what would an average of 95.75 show in terms of the range…” doesn’t make any sense.

A real stretch – If we assume the question implies that the average range is 95.75 then the estimate of the standard deviation based on 20 samples would be 25.6 but that number isn’t close to any of the choices and it still begs the question – what does the phrase “show in terms of the range” mean.

My guess would be that at least one part of the question went missing in action when it was transcribed.

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#201407

MBBinWI
Participant

@rbutler – or the preparer of the question isn’t all that knowledgeable.

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#201413

Robert Butler
Participant

@MBBinWI Actually I started to post a comment with respect to the overall quality of the test and what it might suggest relative to the quality of the training received. I find this question particularly worrisome since it seems to be the second poorly presented question in the material the OP is using.

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#201435

Mike Carnell
Participant

@johnpeters123 That is what happens when you listen to Katie. She has been at this a very long time and knows most of the people who contribute and what will get them to help.

Your sincerity goes a very long way as well.

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#201443

Chris Seider
Participant

@mbbinwi I’m shocked you’d imply someone put together a test question without understanding nuances and breadth of experience. :)

Having created customer “loved” BB, MBB, and GB exams I can tell you it’s not easy or trivial.

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