"bnorris" wrote:
(a-b)/((a+b)/2)*100 = relative percent difference

No. I believe RPD is: abs(a-b) / ( (a+b)/2 ) * 100. See
http://en.wikipedia.org/wiki/Percent_difference .

In Excel, that can be written:

=ABS(A1-B1) / AVERAGE(A1,B1)

formatted as Percentage. Alternatively:

=2*ABS(A1-B1) / (A1 + B1)

again formatted as Percentage.


----- original message -----

"bnorris" wrote in message
...
I'm trying to develop a data control chart and I can't find the function
for
relative percent difference...I think it might be a nested function that I
need to use? I'm new to this aspect of excel so, please be kind. I have
found the formula
ABS (A-B)/A but I need a function for relative percent difference....how
can
I chage the following formula into something that works in excel?

(a-b)/((a+b)/2)*100 = relative percent difference, where a is a data
point
in colum A and b is a data point in colum B?

Thanks