Using Percentiles Based On Column

Hi, joshuar 

First of all, the output result you gave about C should be wrong, correct as shown in the figure.

Second, if you want to calculate by group, you can change the formula PERCENTILEX.INC.

PERCENTILEX.INC function (DAX) - DAX | Microsoft Learn

Measure = SWITCH(
TRUE(),
MAX('RFM Table'[F Value])<= PERCENTILEX.INC(FILTER(ALL('RFM Table'),[Customer Group]=SELECTEDVALUE('RFM Table'[Customer Group])),[F Value],0.2), "1",
MAX('RFM Table'[F Value])<= PERCENTILEX.INC(FILTER(ALL('RFM Table'),[Customer Group]=SELECTEDVALUE('RFM Table'[Customer Group])),[F Value], 0.40 ), "2",
MAX('RFM Table'[F Value])<= PERCENTILEX.INC(FILTER(ALL('RFM Table'),[Customer Group]=SELECTEDVALUE('RFM Table'[Customer Group])),[F Value], 0.60 ), "3",
MAX('RFM Table'[F Value])<= PERCENTILEX.INC(FILTER(ALL('RFM Table'),[Customer Group]=SELECTEDVALUE('RFM Table'[Customer Group])),[F Value],0.80 ), "4", "5")

Best Regards,

Community Support Team _Charlotte

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