CHECK DIGIT

Discussion

Check digit verification is a special check on a numeric or alphanumeric field in which the last (rightmost) character is a check digit. Verification can be either modulus 10 or modulus 11. (You can use REFSPEC, as described in Section 5, to add a check digit to an existing field value in a batch file.)

Modulus checks are used when the risk of error keying in numbers must be reduced to a minimum. Depending on the modulus selected, single digit errors, and single or double transpositions can be checked using a check digit. Modulus 10 detects single transpositions and incorrect keying of a single digit. Modulus 11 detects these, plus double transpositions. (Refer to Appendix D for an exact description of the modulus 10 and 11 checks.)

In general, a check digit is arrived at by performing calculations on a number and then using the result of these calculations as the final digit or "check digit" in that number. For example, suppose a 5-digit charge account number is to be assigned to a new account. The specified calculations (modulus 10 or 11) are performed on the 5-digit number, the result is added as a check digit to the number, and a 6-digit number is assigned as the new account number.

Thereafter, when this number is keyed in and the CDIGIT edit test is selected, then the last digit is checked against the same calculations. If the number was keyed incorrectly, an error is diagnosed.

Note that a number derived using modulus 10 calculations can only be checked by CDIGIT 10, and a number derived using modulus 11 can only be checked by CDIGIT 11.

Letters of the alphabet can be checked by either a modulus 10 or 11 check. Digits are assigned to the letters so they can be treated like numbers. Thus a field with a check digit can contain a mix of numbers and letters, but must not contain any special characters. Any initial plus or minus sign is ignored.