Changing Source column for formula depending on month

[john_london]

Hi guys

So I've nearly finished my document, this last bit is killing me. I've attached what i've done so far, and tried to highlight where my problem is.

Basically, I have this formula in my summary page:

=SUM(SUMIF(Attendance!$I:$I,{"Y","A"},Attendance!$D:$D))

In the Attendance sheet in the formula, column $I:$I refers to October. In the summary page, it currently shows the data for October. What I want to be able to do, is for the user to use the picklist in the Summary page to change the month, which therefore changes the column that the formula references (for example, if they change the picklist to 'November' then the new formula should reference column J):

=SUM(SUMIF(Attendance!$J:$J,{"Y","A"},Attendance!$D:$D))

I've added a table in the PickLists sheet, and put the corresponding column in there, but can't figure out how to do this? Any help would be amazing and would result in my manager thinking I'm ace (or at least not crap).

Cheers,
John