Create a variable to retrieve the data of an prompt

Hi,

I have a query in which I have an indicator which is the number of buildings.

In this query I have two filters: MONTH OF ANALYSIS 1 and MONTH OF ANALYSIS 2

I want to calculate the variation in the number of buildings between MONTH OF ANALYSIS 1 and MONTH OF ANALYSIS 2.

I tried to perform this formula in a variable: = [bat. Number of occupants] Where [Cal. Previous month YYYY.MM] = "psCal. Analysis month 1" But it does not work.

Thank you in advance for your help