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