There's more...

The preceding solution to the problem of finding childless nodes was developed prior to the introduction of the EXCEPT function. The EXCEPT function perhaps makes the calculation somewhat more elegant and easier to understand. To implement the Childless measure using the EXCEPT function, do the following:

1. Create the following measure:

Childless 2 = 
    CONCATENATEX(
        EXCEPT(
            VALUES('R05_Table'[EmployeeKey]),
            VALUES(R05_Table[ParentEmployeeKey])
        ),
        'R05_Table'[EmployeeKey],
        ","
    )

2. On the same Report page, create a second Card visualization and place the Childless 2 measure into the Fields area of the visualization.

In this version, we simply use the EXCEPT function and for the first table provide distinct values for the EmployeeKey column. For the second table, we provide distinct values from the ParentEmployeeKey column. Since childless nodes will not be parents, the table returned by the EXCEPT function is a table of childless nodes, which we can then concatenate using CONCATENATEX.