Calculated Field for New and Missing Inventory Items

I have a dataset of inventory, broken out quarterly. I am trying to display a list of both the new inventory items for a given quarter (i.e., ones that are newly in stock in a selected quarter), and the ones that went out of stock since the previous quarter. So would I achieve this with a calculated field? Maybe something like:

IF ISNULL([Quarterly % Change]) then 'New Inventory' ELSEIF [Quarterly % Change] < -.95 then 'Out of stock' END

Note that Quarterly % change =

(ZN(SUM([Units of Inventory])) - LOOKUP(ZN(SUM([Units of Inventory])), -1)) / ABS(LOOKUP(ZN(SUM([Units of Inventory])), -1))

I think I'm making things more complicated than they need to be, so can this be accomplished more simply?