How to Calculate the unbiased Standard Deviation in Excel using Minitab Formula

F

ferme

#1
does someone know how to calculate the unbiesed standard devistion in excel using the formula as minitab?
 
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Stijloor

Staff member
Super Moderator
#2
Re: How to Calculate the unbiased Standard Devistion in Excel using Minitab Formula

A Quick Bump!

Can someone help?

Thank you very much!!
 

Miner

Forum Moderator
Staff member
Admin
#3
Re: How to Calculate the unbiased Standard Devistion in Excel using Minitab Formula

Do you currently have Minitab? If so, you can find the formulae that Minitab uses using these instructions.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#4
Re: How to Calculate the unbiased Standard Devistion in Excel using Minitab Formula

For Excel, generally the "unbiased" estimate of the standard deviation from a sample is the STDEV formula, versus the STDEVP formula which assumes you know the entire population.

However, there are some more esoteric issues with "unbiased standard deviation" which I found some sniffs of with this Wikipedia article -> Wikipedia reference-linkUnbiased_estimation_of_standard_deviation I do not vouch for this information, I've not run across it elsewhere. Not being a Minitab user, I don't know what Minitab does and (IMO) don't much trust Minitab.
 
F

ferme

#5
Thanks guys, the really issue here is that i know the formula that minitab use, but i am not able to replicate it in excel, i know that i need the C4 factor and all the data, i have it, but the result is not the same in minitab i am doing something wrong but i dont know what. i attached one file that other guy post here i studied the format and can't get the pooled unbised standard deviation, hope someone can help.
 

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E

El Capten

#6
Thanks guys, the really issue here is that i know the formula that minitab use, but i am not able to replicate it in excel, i know that i need the C4 factor and all the data, i have it, but the result is not the same in minitab i am doing something wrong but i dont know what. i attached one file that other guy post here i studied the format and can't get the pooled unbised standard deviation, hope someone can help.
I am not familiar with minitab. I looked at your excel spreadsheet. You can determine your standard deviation from your R Bar value. My text book, "Statistical Quality Control" Grant and Leavenworth 4th Ed. table B page 644 give a d2 factor of 2.326 for Rbar n=5. Formula is Rbar / factor. Your example:
R bar = .931 / 2.326 = .4003 is your unbiased estimate of S'
 
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E

El Capten

#7
I looked at your excel spreadsheet and assume that you are doing a capability study or it represents shop floor data as a control chart.
I used your data and calculated upper and lower control limits for X bar and R bar and see that while your R chart is ok, you have several data points out of control limits for your X bar control chart.
My calculations are based on the reference text noted in my previous reply. UCL X = Xbar +A2Rbar LCL X = Xbar - A2Rbar for n=5, A2 =.58 and for range chart D4 = 2.11

for X bar = 36.858 and R bar = .931 control limits are: UCL = 36.858 + .58 x .931 = 37.398, LCL = 36.858 -.
58 x .931 =36.318 range is Rbar x D4 = .931 x 2.11 = 1.964

I dont know what you application is for this data, but since there are data points outside the control limits, you have an unstable process from which you can not draw any assumptions or predictions about process capability.
 
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