Validation checks
| Presence check | Certain fields such as customer number, item code, quantity etc must be present. |
| Format check (also called picture check) |
e.g. code may have a pattern of 2 letters followed by 4 numbers. The quantity and price must be numeric. |
| Range check | A card expiry date must have a month number between 1 and 12, and date must be after today |
| File lookup check | If the customer has filled in their customer number, the computer can look this up on the customer file and display the name and address. The data entry operator can check that it tallies. |
Check digit check. |
The last digit(s) is calculated from the earlier ones the maths is redone to see it all tallies. (see setail below) |
Batch header checks |
Total number of records in the batch should be calculated by the computer and compared with the figure on the batch header. The control totals and hash totals are also calculated and compared. |
Check digits
Code numbers such as a customer no., employee no. or product no. are often lengthy and prone to error. One way of preventing these errors is to add an extra digit to the end of a code number which has been calculated from the digits of the code number. In this way the code number with its extra check digit is self-checking.
Best-known method of calculating check digits is the modulus-11 system, which traps over 99% of all errors.
- Each digit of the code number is assigned a ‘weight’. The right hand (least significant) digit is given a weight of 2, the next digit to the left 3 and so on.
- Each digit is multiplied by its weight and the products added together.
- The sum of the products is divided by 11 and the remainder obtained.
- The remainder is subtracted from 11 to give the check digit. The two exceptions are:
- If remainder is 0, check digit is 0, not 11
- If remainder is 1, check digit is X, not 10
Example: You will not be asked to recall this example so don't learn it.
To calculate the check digit for the number 1587:
- Original code number 1 5 8 7
- Weights 5 4 3 2
- Multiply digit by its weight 5 20 24 14
- Add products together 5 + 20 + 24 + 14 = 63
- Divide by 11 5 remainder 8
- Sub remainder from 11 11 - 8 =3
Check digit = 3. Complete code number is 15873.
To check that a code number is valid, it is not necessary to recalculate the check digit completely. If the check digit itself is assigned a weight of 1, and the products of the digits (including the check digit) and their respective weights are calculated, their sum will be divisible by 11 if the check digit is correct.
© LEV
