Standard Deviation Expression - Please review

[Gerry]

Thank you very much for your time.

I am trying to calculate the standard deviation of a
range. I have the following expression, but something is
incorrect:

STD DEV: (([SDQT01]+[SDQT02]+[SDQT03]+[SDQT04]+[SDQT05]+
[SDQT06]+[SDQT07]+[SDQT08]+[SDQT09]+[SDQT10]+[SDQT11]+
[SDQT12]))

Where is my mistake?


Thanks very much for your help.

Gerry

 

[Michel Walsh]

Hi,

Well, it is mathematically wrong. The average of a range is not the
average of the averages of the parts of the range, and so is not the
standard deviation too.

Month, Value
1 1
1 2
1 3
2 4



Averages, per month:

Month Average
1 2
2 4

Average of the averages: (2+4)/ 2 = 3

Average of the whole data set (1+2+3+4)/4 = 2.5

Clearly, 2.5 <> 3


You have to compute the average, and the standard deviation, on the initial
samples you have got (or use the formula for computing average from grouped
data). In short, make just one big group, and compute the standard deviation
from it, not from the individual smaller groups making the larger one.



Hoping it may help,
Vanderghast, Access MVP