Check digits

Courses iGCSE Computer Science (2023 - 2025) Data Representation Check digits

An Extra Digit That Checks Entry

Cambridge uses a check digit to catch errors when data is typed or scanned, not when it travels on a cable. An extra digit is calculated from the other digits and stored with them. ISBN numbers and barcodes are the syllabus examples.

At the till, the scanner recalculates the check digit and compares. If they disagree, the code is wrong: a mistyped ISBN or a damaged barcode. It will not catch every swap, but it catches many single-digit mistakes.

A check digit is mainly used to detect errors in

{"answers": {"A": "Data entry such as ISBNs and barcodes", "B": "The fetch-execute cycle", "C": "A CSS colour rule", "D": "CPU cache size"}, "question_text": "A check digit is mainly used to detect errors in", "correct_answer": "A"}

Check Digit versus Parity Bit

Check digit

Calculated from decimal digits of a number you type or scan. Example: the last digit of an ISBN. It is about humans and scanners entering data.

Parity bit

An extra 0 or 1 added to a binary byte so the number of 1s is even or odd. It is about bits after transmission, not about typing an ISBN.

A [[check digit]] is calculated from other digits. [[Parity]] bits check binary after transmission.

{"cloze_text": "A [[check digit]] is calculated from other digits. [[Parity]] bits check binary after transmission."}

{"keyword": "check digit", "definition": "An extra digit calculated from other digits to detect errors when data is entered or scanned"}

Question Type: fill in the blanks

{"paragraph": "ISBNs and [[barcodes]] use a check digit to catch [[entry]] errors."}

Question Type: short answer

{"answer": "The data is rejected / treated as an error, for example a mistyped ISBN.", "rubric": {"0": "Incorrect or no response", "1": "Partial credit", "2": "The data is rejected / treated as an error, for example a mistyped ISBN."}, "question": "What happens if a recalculated check digit does not match the stored one?"}