Sometimes when we report counts we want percent total next to the count. See for instance the Honors Admissions report.
To get a count we use syntax similar to =CountDistinct(Fields!StudentID.Value). Now if we want the distinct count aggregated over a Column or Row Group we use syntax like CountDistinct(Fields!StudentID.Value,"year"). This assumes that name under the Group properties (not the TextBox Properties) is "year". So, to get a percent value we can use the expression:
=CountDistinct(Fields!StudentID.Value)/CountDistinct(Fields!StudentID.Value,"year")