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Topic: How to combine the standard deviations of multiple data subsets
Replies: 6   Last Post: Nov 23, 2012 1:26 PM

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Posts: 1,089
Registered: 12/6/04
Re: How to combine the standard deviations of multiple data subsets
Posted: Oct 4, 1999 5:35 PM
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On Mon, 4 Oct 1999 14:29:17 -0600, "Steve Schnick"
<sschnick@yahoo.com> wrote:
>If one has several subsets of a given data set, and the mean, count,
>and standard deviations for each of these subsets, how can one
>calculate the combined standard deviation of the data subsets? i.e.,
>if the subsets were lumped together into one set, how does one
>calculate this new standard deviation?

Some clues:
1. You need to calculate the new mean. (easy)
2. You should work with variances not standard deviations
3. If a set {Xi} i=1 to n has mean M
then sumof (Xi-k)^2 / n = (k-M)^2 + sumof (Xi-M)^2 / n
Ask again if this is not enough

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