AQA A-Level Computer Science Paper 2, June 2025: Question 10

Mark scheme

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Qu Pt Marking guidance

marks

10 1 Marks are AO1 (understanding) 2

Less storage space required;

A. less memory required

A. reduce file size

NE. less space required

Data can be transmitted more quickly // in less time;

A. less bandwidth required to transmit data

A. reduces cost of sending using a metered connection

NE. easier to transmit

More data/files can be fitted on a storage device with a given capacity (A. size);

A. examples of devices and capacities

A. data/file can be used on a service with a size limit eg email attachments

Max 2

Total

Qu Pt Marking guidance

marks

10 2 Marks are AO1 (understanding) 2

RLE suitable for images (1 mark):

In (some) images it is likely that adjacent pixels will be the same colour;

A. likely to be multiple pixels of same colour in a row as BOD

NE. adjacent values are likely to be the same

Dictionary based suitable for text (max 1 mark):

Repetition will be of words/groups of character/sequences of symbols/substrings;

The repeating data is likely to be spread throughout the text;

A. repeating data is unlikely to be adjacent identical characters/words/groups of

characters

A. never/rarely more than two adjacent letters/words that are the same

How to answer it

Data Compression: Purpose, RLE, and Dictionary Encoding

📌 What this question tests

This question assesses fundamental AO1 knowledge of lossless data compression:

The practical advantages of reducing file size (storage efficiency, transmission speed, and bandwidth consumption).
How structural differences between media types (adjacent repeated pixels in images vs. repeating words/patterns spread across text) determine the suitability of Run Length Encoding (RLE) versus Dictionary-based compression.

Question 10.1 • 2 Marks

Reasons Why Data is Compressed

State two reasons why data is often compressed.

✅ Acceptable Answers (Any Two)

Storage reduction: Takes up less storage space / less memory / reduces overall file size.
Faster transmission: Data can be transmitted more quickly / in less time across a network.
Bandwidth efficiency: Uses less network bandwidth / lowers transmission costs on metered connections.
Device/Service capacity: Allows more files to fit on a fixed-capacity drive, or allows files to meet size limits (e.g. email attachment limits).

❌ Common Errors & Vague Language

"Less space required" – Not Enough (NE). You must state storage space, memory, or file size.
"Easier to transmit" – Not Enough (NE). Too vague; explain why (e.g. "faster transmission", "uses less bandwidth").
Confusing compression with encryption (e.g. claiming it makes data more secure).

🧠 Exam Technique: The Two Pillars of Compression

Whenever an exam asks why compression is used, always separate your thoughts into Storage and Transmission:

Storage: Smaller footprint on secondary storage (SSD, HDD).
Transmission: Less time and less bandwidth required over a network.

Mark Allocation: 1 mark per valid reason stated clearly (Max 2 marks). Marks are AO1 (Knowledge and understanding).

Question 10.2 • 2 Marks

Suitability: RLE for Images vs. Dictionary for Text

Explain why RLE is more suitable for compressing images and dictionary-based methods are more suitable for text.

✅ Mark Scheme Criteria

RLE for images (1 mark):
In images, it is common to have runs of adjacent pixels that are the same colour (multiple consecutive pixels of identical value).
Dictionary methods for text (1 mark):
In text, repetition consists of recurring words, sequences of characters, or substrings that are spread throughout the document (rarely consecutive/adjacent).

💡 Why the Algorithms Work This Way

RLE (Run Length Encoding): Stores a data value alongside a count of consecutive occurrences (e.g. 5 Blue, 12 Red ). If adjacent values are not identical, RLE can actually increase file size due to storing count overheads.
Dictionary (e.g. LZW): Builds an indexed table of repeated tokens (words/phrases) and replaces occurrences anywhere in the file with compact numerical codes.

❌ Common Errors in 10.2

Omitting "Adjacent" / "Consecutive": Saying "images have lots of pixels the same colour" is insufficient without noting they occur in contiguous runs (side-by-side).
Forgetting "Pixels": Writing "adjacent values are the same" without context was rejected. Always contextualise to pixels and colours.
Failing to contrast text structure: Overlooking that English text rarely has consecutive identical words (e.g. you rarely write "the the the"), meaning RLE would fail, whereas dictionary methods locate identical tokens separated across paragraphs.

🧠 Top-Scoring Model Answer

"Images frequently contain large blocks of identical, adjacent pixels of the same colour, which allows RLE to encode them efficiently as single count-colour pairs."

"In contrast, text contains repeating words and substrings distributed throughout the text rather than next to each other, making dictionary methods ideal as they substitute recurring non-adjacent patterns with short pointers."

Mark Allocation: 1 mark for linking RLE to adjacent pixels of the same colour; 1 mark for linking dictionary methods to recurring words/patterns spread non-contiguously throughout text (Total: 2 marks).

Topics

4.5 Fundamentals of data representation · 4.5.6 Representing images, sound and other data