What is the Excel Addin Doing

Question

Hi,
I am just starting to use the excel add in and some of the things I can't figure out.  I am trying to do forecasting, which every demo i have seen looks simple enough.  i have created a set of data using two facts/measures and the first of every month for 2 years.  When i use the table tools, Analyze, Forecast I get different results depending on how i do it.

e.g.
My data (some of)

01/03/2009 00:00	3583973.49	2969
01/04/2009 00:00	3057092.23	2519
01/05/2009 00:00	2053462.79	2382

if i forecast both measures at the same time

01/06/2009 00:00	3948649.77	3014
01/07/2009 00:00	4175897.669	3382
01/08/2009 00:00	3948649.77	3296

if i forecast them on their own

first measure

3992107

3992107

3992107

3992107

3992107

second measure

2776

2988

2631

2708

2476

Can anyone explain why it is doing that?  I am guessing that when you do it together they both impact one another, can i get it so that measures don't influence others?  And why do I only get 3? As opposed 5 when i do them seperately?

A very confused
Matt

Answer 1

Matt

Which version of SQL Server are you using?  2005?

If you are using 2005 then the algorithm used for all time series predictions is ARTxP.  The x is "Cross" and this allows a variable to be influenced by another.  This is very probably the reason you are seeing the difference in values.

The second Q which is "Why does it stop".  Again in 2005 what happens is that if the predictions start to get too wild then SSAS will just stop predicting and you have no way of controlling it.  ARTxP is great at "Short Term/Next Value" prediction but not as good further out.  That is one of the reasons why ARIMA was introduced in 2008.

Allan

Answer 2

Thanks Allan,

We are using 2005, there is currently some arm twisting going on so we can get 2008.  I will add what you say to my case :)