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''' is a data entry quality control method that was originally employed when data records were entered onto sequential 80 column Hollerith cards with an IBM 129 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, the keypunch read a completed card (record) and loaded the 80 keystrokes into a buffer. The data entry operator then reentered the record and the keypunch compared the new keystrokes with those loaded into the buffer from the previous pass. 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.

'''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>

'''TwoThe Pass[[Keypunch#IBM Verification'''056 isCard aVerifier|IBM data056]] entryand quality[[Keypunch#IBM control059 methodCard thatVerifier|059]] wasCard originallyVerifiers employedwere whencompanion datamachines recordsto werethe enteredIBM onto026 sequentialand 80029 columnkeypunches, Hollerithrespectively. cardsThe with anlater IBM 129 Keypunch.keypunch Inalso thecould firstoperate pass throughas a set of records, the data keystrokes were entered onto each card as the data entry operator typed themverifier. OnIn the second pass through thethat batchmode, the keypunchit read a completed card (record) and loaded the 80 keystrokes into a buffer. TheA data entry operator then reentered the record and the keypunch compared the new keystrokes with those loaded into the buffer from the previous pass. 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>

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. This method has survived the Keypunch and is available in some currently available data entry programs (e.g. SPSS Data Entry for Windows). At least one study suggests that single pass data entry with range checks and skip rules approaches the reliability of two-pass data entry (see Controlled Clinical Trials from sometime in the 1990s - Control Clin Trials. 1998 Feb;19(1):15-24.?); however it is desirable to implement both systems in a data entry application.

==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 Keypunchkeypunch and is available in some currently available data entry programs (e.g. [[PSPP]]/[[SPSS]] Data Entry for Windows). 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 quality]]

[[Category:Punched card]]