Calculation between periods

[Anonymous]

Hello, 

I need a help with calculate the difference between 2 dates, for example:

I have this Table:

Periods	Value A	Value B	%Value B
Week 1	680	475	69,85%
Week 2	677	466	68,83%
Week 3	640	438	68,44%
Week 4	646	457	70,74%

And I need a colunm or a measure "Difference" that calculate de difference of colunm "%Value B" by Periods:

Periods	Value A	Value B	%Value B	Difference
Week 1	680	475	69,85%
Week 2	677	466	68,83%	-1,02%
Week 3	640	438	68,44%	-0,39%
Week 4	646	457	70,74%	2,30%

Anybody can help me?

tks.

[Greg_Deckler · Accepted solution]

@Anonymous Add an Index column in Power Query Editor or a Sequential week column like this: Sequential - Microsoft Power BI Community. Then you can use the MTBF pattern. See my article on Mean Time Between Failure (MTBF) which uses EARLIER: http://community.powerbi.com/t5/Community-Blog/Mean-Time-Between-Failure-MTBF-and-Power-BI/ba-p/339586.
The basic pattern is:
Column = 
  VAR __Current = [Value]
  VAR __PreviousDate = MAXX(FILTER('Table','Table'[Date] < EARLIER('Table'[Date])),[Date])

  VAR __Previous = MAXX(FILTER('Table',[Date]=__PreviousDate),[Value])
RETURN
  __Current - __Previous

[Anonymous]

@Greg_Deckler 

thanks for your help, but I dont understand the first pass: 
"VAR __Current = [Value]"

This [Value] is the sum of Value B?

tks

[Greg_Deckler]

@Anonymous In your case yes. The code is generic code, you have to adapt it to your particular table name and column names.