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A test of the reliability of data. Check problems with known results are run through computers to verify correct functioning. (a) A summation check consists of adding a set of figures and using the sum (check sum) to verify accuracy and completeness. (b) A duplication check requires that an operation be repeated and yield identical results when repeated. (c) An echo check is used to verify transmitted data; the data are sent back (echoed) to the source station for comparison with the original data. An echo check is also a technique whereby a computer prints out values of variables and data used in a program. (d) Error-detecting checks search data for forbidden combinations. A parity check is such a check; the number of one bits plus a parity bit must add up to an odd number for odd parity or to an even number for even parity. A longitudinal parity check (LPC), written at the end of a record, does for each track what the parity bit does for each byte. (e) A validity check is a verification that a figure lies within a certain permissible range.