Calculating Percentage Change

Hi There, 

I know this is often asked on the forum though nothing I have found works with what I am wanting to achieve

Currently there is a measure to count how many of each type of correspondence there is depending on the group that is chosen in the slicer. I also have a date slicer choosing which month I want to look at

Now I am wanting to use the measure (count the correspondence for selected month / count of correspondence for previous month ) / count of correspondence for previous month

This will then go into an IF ELSE statement so that if the number is a positive then  the word Positive would show, else Negative

Type        EOM      Group