Help with windows function to calculate running AVG

Hi Nawaz 

Thanks for the reply from Greg_Deckler , please allow me to provide another insight:

Here I create a set of sample:

Then create 2 measures:

SUM_Sales =
VAR _currentCa =
    SELECTEDVALUE ( 'Table'[Category] )
RETURN
    CALCULATE (
        SUM ( 'Table'[sales] ),
        FILTER ( ALLSELECTED ( 'Table' ), 'Table'[Category] = _currentCa )
    )

AVG =
VAR _vtable =
    SUMMARIZE ( ALLSELECTED ( 'Table' ), 'Table'[Category], "_SUM", [SUM_Sales] )
VAR _vtable2 =
    ADDCOLUMNS ( _vtable, "_sort", RANKX ( _vtable, 'Table'[Category],, ASC ) )
VAR _vtable3 =
    ADDCOLUMNS (
        _vtable2,
        "_AVG",
            AVERAGEX (
                FILTER (
                    _vtable2,
                    [_sort] <= EARLIER ( [_sort] )
                        && [_sort]
                            >= EARLIER ( [_sort] ) - 1
                ),
                [_SUM]
            )
    )
RETURN
    MAXX (
        FILTER ( _vtable3, [Category] = SELECTEDVALUE ( 'Table'[Category] ) ),
        [_AVG]
    )

The result is as follow:

Best Regards

Zhengdong Xu
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