Calculate Difference from Previous 'Date' Dynamically

Cool question - using the query editory you can sort the table and add an index so that each row has a distinct number (0,1,2,3, etc).  Here's the formula you want to write ... I broke it out into pieces.

Total = SUM(Total)

Total - Previous Period = VAR thisperiod = MAX(Index)
RETURN CALCULATE ([Total], FILTER (ALL(Table), Index = (thisperiod - 1))

Total - PoP Change = IF( [Total] && [Total - Previous Period], [Total] - [Total - Previous Period] )

Total - PoP Change - Aggregates Correctly = SUMX(Table, [Total - PoP Change] )

With current and previous date calculations we usually use a date table and the date/time functions but those calculations don't work with weeks, so you'd have to do it this way.

No that makes sense - should work fine, here's what you do.

Option 1

1. Undo the sorting and indexing.
2. Right click your query and hit duplicate.
3. In this new query you're going to remove all the columns except the week, then remove the duplicates, sort the column, and add the index.

1. Right click the new query and uncheck the "Enable Load" option
2. In the old query, select the weeks column and click on the merge queries option up in the ribbon. Merge the old query with the new query based using the week column. Left Join is chosen by default - that's what you want.
3. Expand the new column to show the index column.

All measures - the last one gives us the right number for the grand total (i hope that's what it does!)

Elliott Try this...

Total in Previous Period 3 = 
VAR 
	thisperiod = MAX('Table'[Index])
RETURN 
	CALCULATE (
		[Total Measure],
	ALLEXCEPT('Table', 'Table'[Customer]),
	'Table'[Index] = thisperiod - 1) 

I think these are the results you are looking for...