AVERAGE based on columns added into a parameter field

Thanks for danextian's concern about this issue.

Hi, Anonymous 

I am glad to help you.

According to your description, first I created three Measure to calculate the average of different fields:

AvgQS = AVERAGE(Parameter[QA Score])

AvgRPI = AVERAGE(Parameter[RPI])

AvgRSI = AVERAGE(Parameter[RSI])

Then created Fields Parameter with the three fields checked:

At this point my parameter table looks like this:

Then I created a Measure to dynamically fetch the selected parameters while calculating the averages:

example Rank = 
VAR _selectedParameter = CONCATENATEX(VALUES(AverageParameter[AverageParameter Fields]), AverageParameter[AverageParameter Fields], ", ")
RETURN
SWITCH (
    _selectedParameter,
    "'Parameter'[QA Score]", [AvgQS],
    "'Parameter'[RPI]", [AvgRPI],
    "'Parameter'[RSI]", [AvgRSI],
    "'Parameter'[QA Score], 'Parameter'[RPI]",([AvgQS]+[AvgRPI]) / 2,
    "'Parameter'[QA Score], 'Parameter'[RSI]",([AvgQS]+[AvgRSI]) / 2,
    "'Parameter'[RPI], 'Parameter'[RPI]",([AvgRPI]+[AvgRSI]) / 2,
    "'Parameter'[QA Score], 'Parameter'[RPI], 'Parameter'[RSI]",([AvgQS]+[AvgRPI]+[AvgRSI]) / 3
)

I have attached the pbix file for this example below, I hope it helps.

I hope my suggestions give you good ideas, if you have any more questions, please clarify in a follow-up reply.
Best Regards,
Fen Ling,
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