Formula Help - Check on only entry
I need some help with a formula. I want to create a line item (LINE ITEM to check that another line item (LINE ITEM A) only has an entry in one period. The line item that I want to do the check on is a manual input and has a numeric format. I basically want the model to flag an error to tell me if there are values entered in multiple periods in LINE ITEM A (or a value entered in any period other than the one in my view).
My model does not have an 'all periods' field (and I don't want it to) as I'd thought about a formula that checks to see whether the current period in LINE ITEM A is equal to the sum of all periods in LINE ITEM A. I've a feeling that this should be relatively straightforward but I'm struggling currently.