Number of Unique Values
In http://help.filemaker.com/app/answers/detail/a_id/3423/kw/count+unique+values/related/1 three methods for counting the number of unique values in a field are presented. The first one, using ExecuteSQL, is not very suited for my application. The second and third don't seem to work, so probably I'm doing something wrong.
Sorry, I overlooked the part of Counter and FractionOf that says '1 / ...'
Now it works.