---
title: "Spotting a Mistake in Stored or Sent Data"
description: "Data gets corrupted on the way, so systems send extra bits that only fit when nothing has changed. A parity bit or a check digit catches an error without anybody reading the message."
canonical: https://lightmysky.com/learn/computing/spotting-a-mistake-in-stored-or-sent-data-mt_wKwax55eo-
source: https://lightmysky.com/learn/computing/spotting-a-mistake-in-stored-or-sent-data-mt_wKwax55eo-.md
retrieved: 2026-09-12
---

> **Agent view.** This is the Markdown twin of the page, for tools and assistants.
> When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md
> API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md
> Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt
> Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json
> Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`.

# Spotting a Mistake in Stored or Sent Data

Data gets corrupted on the way, so systems send extra bits that only fit when nothing has changed. A parity bit or a check digit catches an error without anybody reading the message.

Subject: Computing · Area: Computing Fundamentals · Ages 13 to 14
Page: https://lightmysky.com/learn/computing/spotting-a-mistake-in-stored-or-sent-data-mt_wKwax55eo-

## Ready when they can

- Add a parity bit to a row, and find the flipped bit in a grid using row and column parity
- Work out the check digit of a short product code and show that one wrong digit fails it
- Say what error detection can and cannot do, using a case where two errors cancel out

## Lesson: Extra bits that catch mistakes

Data rots on the way: a bit flips here, a digit smudges there. So systems send extra bits that only fit when nothing changed, catching an error without anybody reading the message.

**Example.** Give each row an extra parity bit that makes the row hold an even count of ones. In a grid, the row and the column with broken parity cross exactly at the flipped bit.

Product codes do the same with a check digit. The digits 2, 4 and 1 add to 7, so the check digit is 3 to reach 10. Change one digit and the total misses 10, which fails the check.

**Tip.** Know the limit: two flipped bits can cancel and slip past parity. Detection tells you something changed, never that all is well.

**Recap.** Parity and check digits fit only when data is clean, and two cancelling errors can still slip through.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [An Agreed Code: How the Same Bits Mean Letters or Colours](https://lightmysky.com/learn/computing/an-agreed-code-how-the-same-bits-mean-letters-or-colours-mt_IQY-PHtZVZ)