for some fixed modulus .
For example, suppose the number to transmit is 12345 and we have chosen the modulus . The check digit would be 5, and we would transmit it after the digits of the number: 123455. If the receiver received, say, 123445, they would know there had been a mistake since 12344 is not equal to 5 (mod 10). Obviously this will only catch an error in the last digit of the number (or in the check digit), so the choice of as modulus was particularly poor. We’ll see in the next section that a different modulus can catch a fairly high proportion of the most common errors, even using this simple scheme. In general, though, it will at most tell us that there has been an error; it will not help us find where the error was, and cannot be error-correcting.
There are various different errors that can occur when numbers are written, printed or transferred in any manner. Different methods of assigning check digits are better at detecting certain kinds of errors than others. The most common types of errors that occur in practice and their frequencies, according to one study, are as follows: