If a prospect's [Contact ID] becomes their [Person ID] when they become an applicant, then use the data source tab to do a left join from the Marketing table to the Admissions table on these fields. Then use a calculated field to count only those prospects who didn't become applicants:
COUNTD (IF [Contact ID] = [Person ID] THEN NULL ELSE [Contact ID] END)
Thanks for the feedback Eric,
I understand where you are going with the calc. There are some limitations and challenges that won't make your exact suggestion work but I came up with a strategy similar to what you've suggested, joining the two data sources and aggregating all [Contact ID] values to return count of Prospects and then to deriving count of Applicants by aggregating [Contact ID] values that have a specific combination of values form three fields in out of the Admissions data source. I'm taking the time now to verify accuracy of the aggregate data coming through the calc.
Thanks for your time,