SSRS Expression with parameter in WHERE clause

Question

In SSRS, I'm trying to calculate the average number of months a client used a program. The programID is the parameter for the whole report. I'm trying to achieve this (not written with real syntax):

=Avg(Fields!length_of_stay.Value, 0))/30.0 WHERE programid = @ProgramID

Using this question, I came up the the following code which is producing an incorrect answer. I tested in SSMS to get the actual values to compare to SSRS results.

=Avg(IIF(Fields!programid.Value = Parameters!ProgramID.Value, Fields!Length_of_Stay.Value, 0))/30.0

The "/30" is used since the value is in days and I need months. I think the issue is using the parameter value chosen; this is my first report trying to calculate expressions with parameters.

Answer 1

Avg returns the average of all non-null numeric values. 0 is not null so it gets included in the average, distorting your result for every row with a different PragramId. Try using Nothing instead:

=Avg(IIF(Fields!programid.Value = Parameters!ProgramID.Value, Fields!Length_of_Stay.Value, Nothing))/30.0