Using MonthName with Set Analysis

[#1]

Hi, my problem is:

I select a field PeriodEstimate with value Nov 2009.
I've a text with: MONTHNAME(PeriodEstimate-1) and the value is Oct 2009.

Then, I try to use it in a set analysis:

If I write:
=AVG({$< PeriodEstimate ={'Oct 2009'}>} [Result]) works fine.

But, If I write:
=AVG({$< PeriodEstimate ={'MONTHNAME(PeriodEstimate-1)'}>} [Result]) the result is null.

Can you help me? Thanks!!!

[#2 · Accepted solution]

I think you need a dollar sign expansion for the Monthname expression:

=AVG({$< PeriodEstimate =
{'$(=MONTHNAME(PeriodEstimate-1))'}>} [Result])

[#3]

Thanks ninja NMiller!