How to let the count change when two filters changes?

[mingyu]

Hi,

I want to count the distinct subjects only when 'TrialPhase' and 'GroupDose' changes.  I tried the following, but it does not work

COUNT(DISTINCT TOTAL {<TrialPhase, GroupDose>} SUBJECT)

Please help

Thanks

[Ahidhar]

try this 

if(getselectedcount(TrialPhase)>0 and getselectedcount(GroupDose)>0,null(),count(SUBJECT))