[1] Generally this is accomplished by connecting the inventory system with order entry and in retail the point of sale system.
Starting in the 1970s digital computers made possible the ability to implement a perpetual inventory system.
This has been facilitated by bar coding and lately radio frequency identification (RFID) labeling which allows computer systems to quickly read and process inventory information as part of transaction processing.
Perpetual inventory systems can still be vulnerable to errors due to overstatements (phantom inventory) or understatements (missing inventory) that can occur as a result of theft, breakage, scanning errors or untracked inventory movements, leading to systematic errors in replenishment.
Some accountants will add or subtract a value using an adjustment entry (journal voucher), however if all receipts (purchases) and shipments (invoices) are captured as transactions, this would never need to occur.