Retrieve Measures for Previous Event (not date based)

Hi Nathaniel_C 

thanks for your reasponse, I appreacite the help! I can kind of follow what you did, but I am not sure why you did it that way...

A couple of follow-up questions:

If I have a set of other dimension tables linking from the Tickets table (such as Category1, Category2, Classification, Status, many others...) I would ideally like to use these dimensions too whenever I use the new measure for previous event sales. But if I create a new table using summarize like you have done, I am limited in what I can do, is that correct (since the summarize table only contains 2 columns, I would have to add columns to the table for each dimension I wanted to include)?

I had a go at this myself last night, but Im not sure if I am on the right track, here is my DAX:

previousData = 
VAR previousSelectedEventId = SELECTEDVALUE(Event[PreviousEventId])
RETURN
CALCULATE(
	SUM(Tickets[Amount]),
    ALLEXCEPT(Classification, Category1, Category2, Category3),  
    Attendance[EventId] = previousSelectedEventId
)

This appears to work, but my concern is that since I am clearing the filter on Classification, Category1, Category2, and Category3... if my end user uses this measure on another dimension table that I have not included in the ALLEXCEPT function, it will not work as expected... is that correct?