Relative Time Period

[caroline]

Hi,  I have started building a cube in transformer which has measure data based on Monthly reported 'YTD' figures.  So I have three months worth of Data  July to September and each YTD measure is a 'cumulative' total for the year.  What I am required to do is to have a Period value displayed for each month.  I have setup a relative time dimension with the change function used against the "Month" level.  This works but only as long as the previous month had a value.  If the previous month was null or does not exist (in the case of July) then no data is shown in the calculated change column. 

My questions are, is there anyway I can do a if statement to say that if the change is null then just use the current month (I have tried an if statement but it says this is a invalid function).

Also, if I am not doing this the incorrect way please point me in a better direction...