bolligerallstar

Actually, come to think of it, I think I had the same question. However, I was able to decipher (don't remember exactly how because I am past that material right now) that:

Purchases = Ending Inventory + COGS - Beginning Inventory

Which means that the two formulas have different denominators. I had a hard time (and still do!) memorizing some of the formulas in these two volumes because the differences are so subtle.

C3Po -- I don't recall reading that COGS could be used as a proxy for purchases. It would be good if you can point us to the relevant material.