Input validation controls are intended to detect transcription errors in transaction data before they are processed. Since errors detected early are less likely to infiltrate the accounting records, validation procedures are most effective when they are performed as close to the source of the transaction as possible. In the batch system depicted in Figure 9.1, data validation occurs only after the goods have been shipped. Extensive error logs, error correction, and transaction resubmission procedures characterize such systems. By contrast, validity tests performed in real time and POS systems can deal with most errors as they occur.