Queries :: Create A Statistics Function On A Database

I am trying to create a statistics function on a database. The idea is that the user will enter a start and end date and either search for all records during that date range or select a client from a combo box and only view records for that client during the selected dates.

After doing quite a bit of searching, it seems that I should be using a wildcard in the criteria however I cannot get this to work. The code I have been playing with is:


Code:
=Nz(Forms![Statistics]![ClientCombo] ,"*")

I have changed the "*" to a specific client number and if the combo box is left blank, results are shown for that client only and if a client is selected from the combo box then the selected client is shown. The only thing I cannot get it to do is show all entries if the combo box is left blank.