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Does Excel STDEV calculate in 3 stdev (standard deviation)?

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  Post Number #1  
Old 2nd February 2007, 05:14 PM
T Dutton

 
 
Total Posts: 10
Question Does Excel STDEV calculate in 3 stdev (standard deviation)?

Does Excel STDEV calculate in 3 stdev?

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  Post Number #2  
Old 2nd February 2007, 06:24 PM
Scott Catron's Avatar
Scott Catron

 
 
Total Posts: 666
Re: 3 standard Deviation in excel

If I'm interpreting the questions correctly - NO.

STDEV is one standard deviation - you'll have to multiply it by 3 in a formula somewhere.
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  Post Number #3  
Old 2nd February 2007, 06:52 PM
Howard Lee's Avatar
Howard Lee

 
 
Total Posts: 107
Re: 3 standard Deviation in excel

In Excel STDEV yeilds one sample standard deviation. To get 3 sigma you need to multiply it by 3. Also, if you need the standard deviation of a population, you should use STDEVP instead.
  Post Number #4  
Old 2nd February 2007, 07:33 PM
Statistical Steven's Avatar
Statistical Steven

 
 
Total Posts: 984
Re: 3 standard Deviation in excel

Quote:
In Reply to Parent Post by Howard Lee View Post

In Excel STDEV yeilds one sample standard deviation. To get 3 sigma you need to multiply it by 3. Also, if you need the standard deviation of a population, you should use STDEVP instead.
I wish Microsoft would do away with the STDEVP function. It has no use. The difference is that STDEV divides by n-1 and STDEVP divides by n. Can anyone tell me when you would use STDEVP?
  Post Number #5  
Old 2nd February 2007, 10:14 PM
Wesley Richardson's Avatar
Wesley Richardson

 
 
Total Posts: 162
Re: 3 standard Deviation in excel

Quote:
In Reply to Parent Post by Statistical Steven View Post

Can anyone tell me when you would use STDEVP?
Hi Statistical Steven,

It would be used if you had the entire data set. Although this is less common, there are a few cases: 1. The standard deviation of test scores for the entire number of students in a class. 2. The standard deviation of a part characteristic when you have measured all of the parts in the production run. Both cases would typically have a fairly small number in the population, such as less than 100 items.

I accept that if you have measured all of the population values, then you could graph the values and see how they are distributed.

Wes R.
  Post Number #6  
Old 3rd February 2007, 12:29 PM
Steve Prevette's Avatar
Steve Prevette

 
 
Total Posts: 2,378
Re: 3 standard Deviation in excel

Quote:
In Reply to Parent Post by Statistical Steven View Post

I wish Microsoft would do away with the STDEVP function. It has no use. The difference is that STDEV divides by n-1 and STDEVP divides by n. Can anyone tell me when you would use STDEVP?
I think I've also answered this on the ASQ board from a question there. But here goes:

The formula for sigma (STDEVP) is the maximum likelihood estimator for the population standard deviation. However, it is a biased estimator. If you only have a sample of the population (and consider - if the "future" is part of your population, you still only have a sample), STDEVP will have a bias in it. If you repeatedly sample from a population, you will notice that the average STDEVP values will not tend to match the actual population standard deviation. However, if you use STDEV (dividing by n-1), the average of the STDEV's will converge on the population standard deviation.
  Post Number #7  
Old 3rd February 2007, 11:10 PM
Tim Folkerts's Avatar
Tim Folkerts

 
 
Total Posts: 975
Re: Does Excel STDEV calculate in 3 stdev (standard deviation)?

I like what Steve said, but I want to make two minor points.
  1. sigma (STDEVP) would be the population standard deviation, not an estimate. When you have the full set, you don't need to estimate!
  2. I believe it is actually (STDEV)^2 that averages to (STDEVP)^2, not STDEV that average to STDEVP. That is, you take the variances calculated from the sample standard deviations and average them to get an unbiased estimate of the true variance.
Of course, all this is all WAY to esoteric for most people!
  Post Number #8  
Old 4th February 2007, 12:32 AM
Statistical Steven's Avatar
Statistical Steven

 
 
Total Posts: 984
Re: 3 standard Deviation in excel

Actually S^2 is the MLE for sigma squared...but that is not the point. If as Wes stated, you have the entire population, then why would you need to know the standard deviation. That is to say, any inferences to be made can be made from the data in the population. There are no confidence intervals that need to be calculated. I am stumped, regardless of the theoretical differences between sigma and S, why there is an application that someone would be using Excel for that requires the population stdev.
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