How to show aggregated 'count distinct' values?

Hi guys,

I created a simple list with only a few columns, each row represents each units and  the total number of distinct students who enrolled into this unit.

I want to show the aggregated numbers of distinct students for all units at the bottom of the list, it is now using [Sum(DISTINCT_STUDENT_COUNT)] which is just simply adding all the numbers together. However, one studnet can enrolled into 2 units, so even it shows 1 distinct student headcount for unitA and unitB, the total number should be 1 rather than 2.

I tried using CountDistinct at the summary level, but it doesnt work, can someone please help?

Regards,

J

Kind Regards, J