EnglandComputer ScienceSyllabus dot point

How is data made smaller, kept secret, and checked for errors during storage and transmission?

Compression, encryption and error checking: lossy and lossless compression (run-length encoding and dictionary methods), symmetric and asymmetric encryption, and error-detection methods (parity, checksums and check digits).

An Eduqas Component 2 answer on compression, encryption and error checking: lossy versus lossless compression with run-length and dictionary methods, symmetric and asymmetric encryption, and error-detection methods including parity, checksums and check digits.

Generated by Claude Opus 4.814 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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Quick answer

Compression reduces file size. Lossy compression permanently discards data the eye or ear is unlikely to miss (used for JPEG, MP3), giving small files but no perfect restoration. Lossless compression removes only redundancy, so the exact original can be reconstructed (used for text, executables, ZIP). Two lossless methods: run-length encoding (RLE) replaces runs of repeated values with a value and a count; dictionary compression replaces frequently occurring sequences with shorter codes from a dictionary. Encryption keeps data secret. Symmetric encryption uses the same key to encrypt and decrypt, fast but the key must be shared securely. Asymmetric encryption uses a key pair, a public key to encrypt and a private key to decrypt, so the public key can be shared openly, solving key distribution. Error detection checks data was transmitted correctly: a parity bit makes the number of $1$ s even or odd so a single-bit error is spotted; a checksum is a calculated value sent with the data and recomputed on arrival; a check digit is an extra digit appended to a number to catch entry errors.

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What this dot point is asking

Eduqas wants you to distinguish lossy and lossless compression and describe methods (run-length encoding, dictionary), distinguish symmetric and asymmetric encryption, and describe error-detection methods (parity, checksums, check digits). These appear as both short definitions and applied examples.

The answer

Lossy and lossless compression

Key fact

Compression reduces the number of bits needed to store or send data. Lossy compression achieves large size reductions by permanently removing data that humans are unlikely to notice (fine colour detail in images, inaudible frequencies in sound), so the original cannot be perfectly restored, used for photos (JPEG) and music (MP3) where small size matters more than perfection. Lossless compression removes only redundancy, so the exact original can be reconstructed, essential for text, program files and data where any loss would corrupt the file. Run-length encoding (RLE) is a simple lossless method that stores a repeated run as a single value plus a count (ten white pixels become "white, 10"), effective when data has long runs. Dictionary methods replace common sequences with short codes looked up in a dictionary.

Symmetric and asymmetric encryption

Error detection: parity, checksums and check digits

Apply RLE and a parity check

Compress the pixel row "WWWWWBBB" with run-length encoding, then add an even parity bit to the 7-bit code $1010011$ .

step 1: Run-length encode the row

Group consecutive identical values: five Ws then three Bs. RLE stores this as "W5 B3" (value then count), far shorter than the eight separate values.

step 2: Note when RLE helps

RLE shrinks data with long runs (simple graphics, icons); on data with few repeats it can make the file larger, so it is not always beneficial.

step 3: Count the 1s for parity

The code $1010011$ has four $1$ s, which is already even.

step 4: Choose the parity bit

For even parity the total number of $1$ s must stay even, so the parity bit is $0$ , giving $10100110$ . If a single bit flips in transmission, the count becomes odd and the receiver detects the error.

Examples in context

Lossy compression is why JPEG photos and MP3/streaming audio are small enough to send and store; lossless (ZIP, PNG, FLAC) is used where exactness matters. Asymmetric encryption secures the web (HTTPS) and underlies digital signatures, while symmetric encryption does the fast bulk work once a key is agreed. Error detection runs constantly, parity in memory and serial links, checksums in network packets (linking to the data-transmission dot point), and check digits on every barcode and bank-card number. These techniques connect representation, transmission and the data-protection issues from Component 1.

Try this

Q1. Give one situation where lossless compression must be used rather than lossy. [1 mark]

Cue. Compressing a program executable or a text document (or any file where losing data would corrupt it).

Q2. In asymmetric encryption, which key is used to encrypt and which to decrypt? [2 marks]

Cue. The public key encrypts; the matching private key decrypts.

Q3. For even parity, what parity bit is added to the code $1101000$ ? [1 mark]

Cue. The code has three $1$ s (odd), so the parity bit is $1$ to make the total even.

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas 20206 marksExplain the difference between lossy and lossless compression, describe how run-length encoding works, and give one situation where lossless compression must be used.

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Lossy versus lossless (up to 2 marks): lossy compression permanently removes some data (detail the human eye or ear is unlikely to miss) to achieve a much smaller file, so the original cannot be perfectly restored; lossless compression removes only redundancy so the exact original can be reconstructed.

Run-length encoding (up to 2 marks): a lossless method that replaces runs of the same repeated value with a single value and a count, for example a row of $10$ identical white pixels is stored as "white, 10" rather than ten separate values; it works well on data with long runs.

Lossless required (up to 2 marks): any situation where exact data matters, such as compressing a program executable, a text document, or a spreadsheet, where losing any data would corrupt it.

Markers reward the permanent-loss versus exact-restoration distinction, the value-plus-count description of RLE, and a valid lossless-required example (executables, text, where accuracy is essential).

Eduqas 20216 marksExplain the difference between symmetric and asymmetric encryption, and describe how a parity bit is used to detect an error in transmission.

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Symmetric encryption (up to 2 marks): the same single key is used to both encrypt and decrypt the data; it is fast, but the key must be shared securely with the recipient, which is the main difficulty.

Asymmetric encryption (up to 2 marks): a pair of keys is used, a public key to encrypt and a private key to decrypt (or vice versa for signing); the public key can be shared openly while the private key is kept secret, solving the key-distribution problem.

Parity bit (up to 2 marks): an extra bit added to a binary code so that the total number of $1$ s is even (even parity) or odd (odd parity); the receiver counts the $1$ s and, if the parity is wrong, knows an error occurred during transmission. It detects single-bit errors but not all multi-bit errors.

Markers reward the same-key versus key-pair distinction, the public/private roles, and the parity-bit count-the-ones check with its limitation.

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Sources & how we know this

WJEC Eduqas GCE AS/A Level Computer Science specification (from 2015) — Eduqas (2015)