Beginning of Period / End of Period (BOP/EOP)

Beginning-of-period (BOP) and end-of-period (EOP) inventory are the stock positions that open and close each period of a merchandise plan, linked by the identity EOP = BOP + intake − sales − markdowns. Each period’s EOP is the next period’s BOP, which chains every period in the season to the ones before it.

The identity is bookkeeping, not a model: whatever a period opens with, plus what lands, minus what sells and what is marked down, is what it closes with. Its force comes from the chaining — because each EOP carries forward as the next BOP, no period stands alone, and changing any one line in any period moves every later stock position. The WSSI is this identity run at weekly grain; a monthly merchandise plan is the same identity at monthly grain, where the positions are usually called BOM and EOM; and open-to-buy is the identity solved for receipts — planned receipts = planned sales + planned markdowns + planned EOP − planned BOP.

The chain is why one bad week is never just that week. Miss sales and the unsold units do not vanish — they roll into the next BOP, on top of receipts that were planned against a smaller opening. Cover lengthens, the surplus carries forward, and it keeps carrying forward until something absorbs it: intake comes down through the open-to-buy, or the stock is repriced through markdown. Whether the miss ends as a small intake correction or a season-scale markdown depends largely on how quickly it is seen — which is the argument for tracking the BOP/EOP chain weekly in a WSSI rather than discovering the drift at month end.