how do I use a formula that subtracts Col B from Col A and divides by Col C but on a per month basis

[#1]

I have 3 months of data in my file, Jan, Feb and Mar.

I'm using this formula 

(Col Sum( :A ) - Col Sum( :B )) / Col Sum( :C )

But I need the value per month.

Thanks

[#2 · Accepted solution]

Most Col Stat() functions support optional by-variables. 

This formula, where :Month is the by-variable, give monthly values (repeated in every row of each month)

(Col Sum( :A, :Month ) - Col Sum( :B, :Month )) / Col Sum( :C, :Month )

[#3]

Perfect, thanks!