=SUM(INDEX(C2:Z2,MATCH(B2,C1:Z1,0)-12):INDEX(C2:Z2,MATCH(B2,C1:Z1,0)))

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HTH

Bob

<yeen yeen wrote in message ...
Hi there,

I have Names in column A starting A2, and Starting Month (mmm yyyy) in
column B starting B2. Across the top starting C1:Z1 I have 24 months (Jan
07 - Dec 08). Let's say I have few thousand records and I will fill in
data from C2:Z5000.

In column AA, I need 12 months average prior to the starting month, i.e.
if Starting month is Jul 08, I need the average of Jul 07 till Jun 08 for
each row.

If I change the starting month then the average will change to capture the
12 months prior to that... pls help!! Thanks!!!