Cambridge O Level Computer Science · Syllabus 2210 · Data Transmission
Check Digit
What is Check Digit?
A check digit is an extra digit calculated from the other digits of an identifier and stored with them; the system recalculates it whenever the identifier is entered or scanned and compares the two values to detect data-entry errors.
This definition is part of the Data Transmission chapter in Cambridge O Level Computer Science.
Check Digit in context
A check digit is an extra digit calculated from the other digits of an identifier and placed with them, normally at the end. Whenever the identifier is later entered or scanned, the receiving system recalculates the check digit from the digits supplied and compares it with the check digit that was supplied. If they disagree, the identifier has been entered incorrectly. Check digits are used with international standard book numbers (ISBN) and with bar codes. Their job is to catch errors in data entry — a mistyped digit, a misread bar code, two digits swapped — not errors that occur during transmission across a network.
Common mistakes with Check Digit
- Trap. A check digit shows only that a number is internally consistent with its own calculation. It does not prove the identifier corresponds to a real book or a real product.