Skew Coefficient

You can come close with the following approach:

In this example, we have a list of 10 inputs in column A.

I have found the skew using the expression found in the Excel help site for the Skew function:

I found the deviation with:

=STDEVP($A)

and the mean with:

=AVERAGE($A)

The expression being summed in the expression above was calculated for each entry using:

=(($A-B$2)/B$1)^3

The summation and multiplication in the footer row is:

=(COUNT($A)/((COUNT($A)-1)*(COUNT($A)-1)))*SUM(B)

My result is within about 4% of what the help page says Excel would calculate. Not too bad for this sort of operation. I'm not sure where the difference would be, but I would be happy to get that close.

Jerry

Hello Jerry,

The formula

=(COUNT($A)/((COUNT($A)-1)*(COUNT($A)-1)))*SUM(B)

should be

=(COUNT($A)/((COUNT($A)-1)*(COUNT($A)-2)))*SUM(B)

And you need to use STDEV() instead of STDEVP() in order to get the same result as in

http://office.microsoft.com/en-001/excel-help/skew-HP005209261.aspx

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Perhaps we might exploit Numbers a little harder without using intermediate variables for small data set like this one.

A1  data
A2  3
A3  4
A4  5
A5  2
A6  3
A7  4
A8  5
A9  6
A10 4
A11 7

B1  ((x-avg(X))/std(X))^3
B2  =(($A2-AVERAGE($data))/STDEV($data))^3
B3  =(($A3-AVERAGE($data))/STDEV($data))^3
B4  =(($A4-AVERAGE($data))/STDEV($data))^3
B5  =(($A5-AVERAGE($data))/STDEV($data))^3
B6  =(($A6-AVERAGE($data))/STDEV($data))^3
B7  =(($A7-AVERAGE($data))/STDEV($data))^3
B8  =(($A8-AVERAGE($data))/STDEV($data))^3
B9  =(($A9-AVERAGE($data))/STDEV($data))^3
B10 =(($A10-AVERAGE($data))/STDEV($data))^3
B11 =(($A11-AVERAGE($data))/STDEV($data))^3

C1  skew
C2  =COUNT($data)/((COUNT($data)-1)*(COUNT($data)-2))*SUM('((x-avg(X))/std(X))^3')

Kind regards,

H

Numbers does not have a skew function. You can use the other stastical functions in Numbers to calculate skew.

H,

Thanks, it was my intention to write the expression as you did (with the -2), but it was an entry error. Don't know how I missed that.

Jerry