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'''Two-pass verification''', also called '''double data entry''', is a data entry quality control method that was originally employed when data records were entered onto sequential 80-column [[Hollerith card]]s with a [[keypunch]]. In the first pass through a set of records, the data keystrokes were entered onto each card as the data entry operator typed them. On the second pass through the batch, an operator at a separate machine, called a ''verifier'', entered the same data. The verifier compared the second operator's keystrokes with the contents of the original card. If there were no differences, a verification notch was punched on the right edge of the card.<ref>{{cite web | url=http://www.museumwaalsdorp.nl/computer/en/punchcards.html | title=Everything about punch cards | date=December 28, 2012 | accessdate=June 20, 2013}}</ref>
The [[Keypunch#IBM 056 Card Verifier|IBM 056]] and [[Keypunch#IBM 059 Card Verifier|059]] Card Verifiers were companion machines to the IBM 026 and 029 keypunches, respectively. The later IBM 129 keypunch also could operate as a verifier. In that mode, it read a completed card (record) and loaded the 80 keystrokes into a buffer. A data entry operator reentered the record and the keypunch compared the new keystrokes with those loaded into the buffer. If a discrepancy occurred the operator was given a chance to reenter that keystroke and ultimately overwrite the entry in the buffer. If all keystrokes matched the original card, it was passed through and received a verification punch. If corrections were required, then the operator was prompted to discard the original card and insert a fresh card on which corrected keystrokes were typed. The corrected record (card) was passed through and received a corrected verification punch.<ref>{{cite web| url=http://ed-thelen.org/comp-hist/IBM-ProdAnn/129.pdf
==Modern use==
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