Calculating a weighted average with the AVEDEV function

[SPI OCU]

Hello everyone...

I have used the AVEDEV function to calculate an average deviation of calls that are taken vs. forecasted for our business. I have done this for each dept that we are tracking with a year-to-date AVEDEV & a month-to-date AVEDEV. The question I have is if this is an Average Deviation of calls that are taken vs forecasted, then how would I calculate an average of all those depts combined? If Im not mistaken, this would be like averaging an average and would not provide me a "true" average deviation. How would I calculate a weighted average deviation? (If that makes sense)

A B C
Taken Forecasted
1) 1000 1565 (SEE FORMULA BELOW)
2) NA NA
3) 2185 2354
4) 2546 2648
5)
6) 1254 1865

This is used to get an absolute number (similar to the ABS function) and is in C1:C6 ....

=IF(A1="NA","NA",IF(B1="NA","NA",IF(A1="","",IF(A1-B1>0,A1-B1,B1-A1))))

This is used to get the average deviation of the results of the formulas in the C column. This formula would be in C7.

=IF(ISERROR(AVEDEV(C1:C6)),"",AVEDEV(C1:C6))

Now this is for a single dept. I have apprx 13 depts that are being calculated this way, and I would like to get an AVEDEV of those AVEDEV numbers.

I apologize if this is long-winded, but Ive just started to use this function and would like to be sure Im reporting accurate data.

Thanks - SPG OCU