Minimum for ALL

Hi newgirl 
Firstly, it is recommended to disable the automatic date hierarchies and create a calendar table that is connected to your transactions table.
This will allow for much more efficient and flexible work.

After you have this model you can use the formulas :

step1_avg = AVERAGE('Table'[Step 1])

and for minimum months average :

min_average =

 if([step1_avg]<>BLANK(),CALCULATE(

Minx(SUMMARIZE(VALUES(caledar[monthYear]),caledar[monthYear],"_x",[step1_avg]),[_x]),ALLSELECTED('caledar')),BLANK())

Result:

The pbix is attached

more information about creating date table here :
https://iterationinsights.com/article/how-to-create-a-date-table-in-power-bi/

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