SSRS MDX for Data from previous two weeks - sql-server

Question

I am trying to get the data fromt the present calender week to the previous two weeks. If I specify the date range as parameters in the MDX query, the table gets filled up properly.
SELECT NON EMPTY {[Measures].[planned_cumulative],[Measures].[Last Forecast CW] } ON COLUMNS,
NON EMPTY { ([Project].[Projekt-Task].[Task].ALLMEMBERS* { [Date].[Date - CW].[Week].&[2014]&[201420] : [Date].[Date - CW].[Week].&[2014]&[201422]})} ON ROWS FROM [DWH]
But if I try and use the lag function, I get an error. Here is the MDX query.
SELECT NON EMPTY {[Measures].[planned_cumulative],[Measures].[Last Forecast CW] } ON COLUMNS,
NON EMPTY { ([Project].[Projekt-Task].[Task].ALLMEMBERS* [Date].[DATECW - CW].[Week].CurrentMember.Lag(2) ) } ON ROWS
FROM [DWH]

Answer 1

Your first query (quite rightly) identifies a range of two weeks, by using two members from the dimension separated by a colon i.e. [Week X] : [Week Y].
Your second query uses the LAG function, which will return a member on your dimension, not a range of members.
Try this:
SELECT NON EMPTY {[Measures].[planned_cumulative],[Measures].[Last Forecast CW] } ON COLUMNS,
NON EMPTY {[Project].[Projekt-Task].[Task].ALLMEMBERS * TAIL([Date].[Date - CW].[Week],3) } ON ROWS
FROM [DWH]
Also, your first query refers to [Date].[Date - CW].[Week] whereas your second query uses [Date].[DATECW - CW].[Week]. I have gone with the first option as you say that query works.
Let me know if that helps.
Ash
EDIT: Apologies for that. I have amended the query to use the TAIL function, which will give you a number of tuples from a set starting at the end and working backwards. So with TAIL ({Set Of Weeks},3) this should give you the current week and the previous two weeks. I have tested a similar query on my cube and it seems produce the expected results.
Hope that solves it for you.
Ash