这两个inventory system是用来track the quantity of goods on hand的两种方法。
周期系统periodic system： 依靠临时的实物计数，也就是physical count of the inventory 去determine the ending inventory balance and the COGS,期末库存余额和销货成本。Periodic 没有一个直观的COGS账户，需要你通过计算来得出COGS，如何计算COGS呢： COGS is calculated in a lump sum at the end of the reporting period, by adding total purchases to the beginning inventory and subtracting ending inventory. This means it can be difficult to obtain a precise cost of goods sold figure prior to the end of the reporting period.有的公司货物可能上千上万所以可能只会一个季度查一次或者一年查一次，如此一来，那可能这个公司的inventory和COGS account figures 就没有及时更新，不fresh or accurate。
In contrast, the perpetual system keeps track of inventory balances continuously, with updates made automatically whenever a product is received or sold. Purchases and returns are immediately recorded in the inventory account. As long as there is no theft or damage, the inventory account balance should be accurate. The cost of goods sold account is also updated continuously as each sale is made.
永续系统perpetual system: keeps continual track of inventory balances. 持续跟踪库存余额。这个不像periodic会有一个COGS account。会随着inventory-related transaction的发生持续更新general ledger或者inventory journal。
Opening Inventory + Purchases - Closing Inventory = COGS
2018-03-29
