Two-pass verification: Difference between revisions

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[[File:Keypunching at Texas A&M2.jpg|thumb|right|Two women entering data onto punched cards at Texas A&M in the 1950s. The woman at the right is seated at an [[IBM 026]] keypunch machine. The woman at left is at an [[Keypunch#IBM 056 Card Verifier|IBM 056]] Card Verifier. She would re-enter the data and the '056 verifier machine would check that it matched the data punched onto the cards.]]

'''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 noneno 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] | title=IBM 129 Data Card Recorder | access-date=2024-02-08}}</ref>

==Modern use==

While this method of quality control clearly is not proof against systematic errors or operator misread entries from a source document, it is very useful in catching and correcting random miskeyed strokes which occur even with experienced data entry operators. However, it proved to be a fatally tragic flaw in the [[Therac 25]] incident. This method has survived the keypunch and is available in some currently available data entry programs (e.g. [[PSPP]]/[[SPSS]] Data Entry). At least oneOne study suggests that, singleat least in simulated circumstances, a single-pass data entry with range checks and {{clarify|text=skip rules|date=October approaches2024}} might not significantly alter the reliability of outcomes compared to studies using two-pass data entry;<ref>{{cite (seejournal Controlled|last1=Day Clinical|first1=Simon Trials|title=Double fromData sometimeEntry: inWhat theValue, 1990sWhat -Price? Control|journal=Controlled ClinClinical Trials. |year=1998 Feb;|volume=19( |issue=1):15-24 |pages=15–24 |doi=10.?1016/S0197-2456(97);00096-2 |pmid=9492966 }}</ref> however, it is desirable to implement both systems in a data entry application.

==References==

{{Reflist}}

{{DEFAULTSORT:Two Pass Verification}}

[[Category:Data collection]]

[[Category:HistoryData of computingquality]]

[[Category:DataPunched collectioncard]]