Calculate value from previous month

[faustusxanthis]

Hello! 

I am currently stuck with this problem and I can't seem to find the solution anywhere. 

I currently have a table with 3 columns that look like this:

I would like to have a calculated column such that it gives the difference of volume from the previous month so it looks like this: 

Thanks in advance!

[faustusxanthis · Accepted solution]

I got it. I used LOOKUPVALUE. 

=[Volume]- LOOKUPVALUE([Volume], [Reason], [Reason], [Month], PREVIOUSMONTH([Month]))