- Two pass verification
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 cards 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. 
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. 
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 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.
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