How to return a median from a calculated table using ADDMISSINGITEMS.

Hi BradBuske 

On the immediate issue:

A DAX query must return a table, not a scalar value, which is the reason for the error message.

If you want to see the value of a scalar expression using a DAX query, you must include it in a table of some sort.

For example, you could convert your current expression into a table with this query:

EVALUATE
VAR MedianValue =
    MEDIANX (
       // ...
    )
RETURN
    { MedianValue } -- use simple table constructor

    -- Alternatively use ROW function
    -- ROW ( "Median", MedianValue ) 

However, a measure to be used in Power BI visuals must return a scalar value, so your final measure could be:

Median Measure =
MEDIANX (
    // ...
)

Use of SUMMARIZECOLUMNS and ADDMISSINGITEMS:

I would generally be cautious of using SUMMARIZECOLUMNS in measures (see here). It may work in some scenarios but can throw errors if referenced within other measures e.g. within iterators with context transition.

A possible re-writing of your measure with a few other tweaks to give the same result.

(I have not been able to test so may require some tweaking):

MedianMeasure =
CALCULATE (
    MEDIANX (
        -- CROSSJOIN will return all combinations regardless of
        -- existence in fact table
        CROSSJOIN (
            VALUES ( 'DATE - CENSUS'[FULL DATE] ),
            VALUES ( 'CARE TEAM - CENSUS'[CARE TEAM NAME] )
        ),
        COALESCE ( [Hospital Encounter Count], 0 )
    ),
    -- Wrap filters in KEEPFILTERS in order to intersect with existing filters
    KEEPFILTERS ( 'CARE TEAM - CENSUS'[CARE TEAM NAME] = "Adolescent Medicine" ),
    KEEPFILTERS (
        NOT 'DATE - CENSUS'[FULL DATE] IN { DATE ( 1900, 1, 1 ), DATE ( 2099, 12, 31 ) }
    )
)

Regards