Maybe you intended to do something like this ?

Assuming your table as posted is in A1:E4
Beginning with 2 related variables,
ie a product: Prod 3,
and a certain known value for the product: 57

To get that product's value in the table just to the left of that known value:
=INDEX(OFFSET(B1:E1,MATCH("Prod
3",A2:A4,0),),MATCH(57,OFFSET(B1:E1,MATCH("Prod 3",A2:A4,0),),0)-1)

p/s: Don't use all caps even in your subject line
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"Morton Detwyler" wrote:
I have rows of product data with monthly column headings. Each month, new
data is added. I already have the current month number, but need to extract
the previous month's number but can not use the monthly headings as a
reference. If I have the number "57" for Prod 3, from Nov. 08, what formula
can be used to find the number "12" also from Prod 3, from Oct. 08. Again, I
can not use the monthly column headings as a reference for extraction. Thanks
much for the help!

Aug 08 Sep 08 Oct 08 Nov 08
Prod 1 14 16 9 17
Prod 2 23 42 37 14
Prod 3 37 16 12 57