Subtract values from same column based on multiple filters

Hi, DreamToGet 

Based on your description, I created data to reproduce your scenario. The pbix file is attached in the end.

Table:

In Power Query, you may create a new query with the following m codes in 'Advanced Editor'.

let
    Source = Table.Group(Table, {"DeviceSerialNo", "TestStep"}, {{"Data", each 
let 
x=try Number.From([Output]{0}) otherwise null,
y=try Number.From([Output]{1}) otherwise null
in x-y , type number}})

in
    Source

Result:

If you want to use DAX, you need to create an index column in Power Query.

You may create a measure as below.

Result Measure = 
SUMX(
    SUMMARIZE(
        'Table',
        [DeviceSerialNo],
        [TestStep],
        "Result",
        var minindex = MIN('Table'[Index])
        var maxindex = MAX('Table'[Index])
        var val1 = 
        IFERROR(
            VALUE(
                MAXX(
                    FILTER(
                        'Table',
                        [Index]=minindex
                    ),
                    [Output]
                )
            ),
            BLANK()
        )
        var val2 = 
        IFERROR(
            VALUE(
                MAXX(
                    FILTER(
                        'Table',
                        [Index]=maxindex
                    ),
                    [Output]
                )
            ),
            BLANK()
        )
        return
        val1-val2
    ),
    [Result]
)

Result:

Best Regards

Allan

If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.

Hi, DreamToGet 

In Power Query, you may add a new step with the following m codes. The pbix file is attached in the end.

= Table.Group(#"Changed Type", {"DeviceSerialNo", "TestStep"}, {{"Data", each 
let 
x= Table.Max( Table.SelectRows(_,each [TestProcess]="Process-Pre"),"StartDate")[Output],
y= Table.Max( Table.SelectRows(_,each [TestProcess]="Process-Post"),"StartDate")[Output],
m= try Number.From(x) otherwise null,
n= try Number.From(y) otherwise null
in m-n
}})

Result:

Best Regards

Allan

If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.