Use check digits to catch data-entry errors
Explain how a check digit is calculated from other digits and checked in ISBNs and bar codes.
Section 2 · Data transmission
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content sections 2.1–2.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a check digit calculated from?
What you will learn
- Explain the role of a check digit
- Describe how the entered code is checked
- Identify ISBNs and bar codes as uses
A check digit tests an entered code
A check digit is calculated from the other digits in a code using a defined rule. When the code is entered or scanned, the system recalculates the check digit and compares it with the one supplied.
Apply and check the rule
If they do not match, the entry may contain an error and can be rejected or checked again. ISBNs and bar codes use check digits. A match detects some common mistakes but does not prove that the entire code is correct, and it does not automatically repair it.
Pause and explain
A check digit does not match after a barcode scan. What can the system conclude?
Worked example
A scanned bar code has a check digit that fails the system's calculation. What should the system do?
Show the worked solution
- Recognise a possible data-entry or scanning error.
- Reject or flag the code for another scan or manual check.
- Do not guess the intended digits from the failed check alone.
Answer Flag or reject the code and request another scan or check; the check digit detects a possible error but does not correct it.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how a check digit helps when entering an ISBN.
Give me a hint
Describe calculate, compare and respond.
Compare my reasoning
- The check digit is derived from the other ISBN digits.
- The system recalculates it from the entered digits.
- A mismatch signals a possible typing error.
The system recalculates and compares the check digit; a mismatch indicates a possible entry error.
Look for these in your work
- Derived value stated
- Recalculation stated
- Mismatch interpreted
A bar code scans clearly but its check digit fails. Give one appropriate next action.
Give me a hint
The code may contain an error even if the image was readable.
Compare my reasoning
- Flag or reject the code.
- Rescan or ask for manual verification.
- Do not silently substitute an assumed digit.
Rescan the bar code or check the code manually; the failed digit check is evidence of a possible error.
Look for these in your work
- Flagging action
- Recovery action
- No unsupported correction
A retailer says that a matching check digit proves a product code is completely correct. Explain why this is too strong.
Give me a hint
Think about the information the check digit verifies and the errors it may miss.
Compare my reasoning
- The check digit tests a calculated relationship among digits.
- Some different incorrect codes can still satisfy the same check rule.
- A matching check reduces certain entry errors but is not proof that every field is valid.
A match only shows that the code passes the check-digit rule. Some errors can go undetected, so it does not prove complete correctness or authenticity.
Look for these in your work
- Check rule explained
- Limit stated
- Claim corrected
Common mistakes
- Calling a check digit an error-correction code
- Assuming every possible wrong code will fail the check
What can you explain now?
Name two examples of codes that use check digits in the syllabus.
Compare with the explanation
ISBNs and bar codes
Check digits help detect errors when these codes are entered or scanned.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
What does a failed check digit tell a system, and what does it not tell the system?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.