Mean Period Change = 
VAR _ChangesForContext = 
    FILTER (
        ADDCOLUMNS ( ALL ( <<DateTable>>[<<DateColumn>>] ), "@Change", [Period Change] ),
        NOT ISBLANK ( [@Change] )
    )
RETURN AVERAGEX ( _ChangesForContext, [@Change] )
 

StdDev Period Change = 
VAR _ChangesForContext = 
    FILTER (
        ADDCOLUMNS ( ALL ( <<DateTable>>[<<DateColumn>>] ), "@Change", [Period Change] ),
        NOT ISBLANK ( [@Change] )
    )
RETURN STDEVX.S ( _ChangesForContext, [@Change] )