How to do Running Total on a field

I am creating a Summary report and on one of the fields "Resume Exists" has values of Yes or No. I am trying to do a summary by BU on this field to say how many resumes don't exist. So I would like it to total how many No's are in the Resume Exists field.

I was thinking of having the field on the detail row, but hide it, then do a summary on the footer.

I have not been very successful and was wondering if anyone has done this and if you can provide me an example. Any help would be greatly appreciated.

If you just want to get a total per group (BU), you don't need to use running total. Just add an aggregate on the group level like this: =Sum(IIF(Fields!ResumeExists.Value="No", 1, 0)). However, if you want to have a running total in the detail row to show how many No's you've encountered so far in each BU, you need to use the running value function: =RunningValue(IIF(Fields!ResumeExists.Value="No", 1, 0), Sum, "BUGroup").