Possible to calculate standard deviation based on a matrix summary column?

BeaudryL 

Please use this measure:

STD Dev = 
VAR __MeanByClinician = 
    ADDCOLUMNS(
        SUMMARIZE( MENTORS , MENTORS[Clinician] ),
        "@Mean" , CALCULATE( AVERAGE( MENTORS[Response.Value] ) )
    )
VAR __StdDev = STDEVX.P( __MeanByClinician , [@Mean] )
RETURN
    __StdDev