Two-pass verification

Two-pass verification, also known as double data entry, is a quality assurance technique that was initially developed for inputting information onto sequential 80-column Hollerith cards using a keypunch. During the initial pass, a data entry clerk punched the keystrokes onto each card. In the subsequent pass, a different operator employed a verifier device to re-enter the identical data, which was then cross-checked against the original card. When no discrepancies were detected, a verification notch was punched along the card's edge.

The IBM 056 and 059 Card Verifiers were designed to work alongside IBM 026 and 029 keypunch machines, respectively. Additionally, the IBM 129 keypunch served as a verifier by reading finished cards into a buffer and comparing newly entered keystrokes with the stored data. Any inconsistencies triggered card correction or replacement, with verified or amended punches applied as needed.

Modern use

While two-pass verification is effective at identifying random keystroke mistakes frequently made even by skilled operators, it does not fully prevent systematic errors or misinterpretations originating from source documents. These limitations have been linked to incidents such as the Therac 25 accident. Despite its origins in keypunch technology, this verification approach continues to be used today and is incorporated into certain modern data entry applications (for example, PSPP/SPSS Data Entry). Research indicates that single-pass data entry combined with range validations and skip logic can achieve reliability comparable to two-pass methods in some situations; however, employing both strategies is generally recommended to maximize data accuracy.