AQA A-Level Computer Science AS Paper 2, June 2025: Question 5

2 marks · Easy difficulty · Short Answer

Explain why digital files are compressed and determine the run length encoding (RLE) representation for a given sequence of characters.

Practise this question

Question

Question 05.1 asks to explain why digital files are often stored in compressed format, worth 1 mark. Question 05.2 states that run length encoding (RLE) can be used to compress data and displays Figure 2 showing the sequence of characters: 'B B G G G R R R R W'. It asks students to state the RLE for the data shown in Figure 2, worth 1 mark.
Question text

05.1 Explain why digital files are often stored in compressed format.

[1 mark]

05.2 Run length encoding (RLE) can be used to compress data. Figure 2 shows some of

the data from a file.

Figure 2

BBGGGRRRRW

State the RLE for the data shown in Figure 2.

[1 mark]

Mark scheme

Show the mark scheme Mark scheme for Question 05.1 awards 1 mark (AO1) for either: 'To reduce storage requirements // so the files take up less storage space // to store more files per amount of storage space' or 'To allow faster transmission of the files (across a network)'. For Question 05.2, 1 mark (AO2) is awarded for '(B, 2) (G, 3) (R, 4) (W, 1)'. Notes allow frequency and value in either order if consistent, and ignore formatting such as brackets, spaces, and commas (e.g. B2 G3 R4 W1 is accepted).

Qu Pt Marking Guidance Marks

05 1 Mark is for AO1 (understanding) 1

To reduce storage requirements // so the files take up less storage space // to store

more files per amount of storage space;

To allow faster transmission of the files (across a network);

Max 1

Qu Pt Marking Guidance Marks

05 2 Mark is for AO2 (application) 1

(B, 2) (G, 3) (R, 4) (W, 1);

A. Frequency and value either way around if consistent

I. Brackets, spaces and commas, eg B2 G3 R4 W1 is correct.

How to answer it

Data Compression & Run Length Encoding (RLE)

📋 WHAT THIS QUESTION TESTS

This question evaluates your foundational knowledge of data representation and compression techniques in AQA AS Computer Science:

  • AO1 (Understanding): Explaining the primary computational purposes and advantages of file compression.
  • AO2 (Application): Applying the Run Length Encoding (RLE) algorithm directly to a given raw sequence of repeating values.
QUESTION 05.1 • 1 MARK

Purpose of File Compression

Explain why digital files are often stored in compressed format.

✅ Acceptable Answers (Any 1)

  • Storage reduction: To reduce required storage space / so files consume less capacity on secondary storage / allows more files to be stored.
  • Transmission speed: To allow faster transmission or download speeds across a network/Internet.

💡 Key Knowledge

Compression reduces the number of bits needed to store or transmit data.

  • Storage impact: Lower capacity requirements on hard drives, SSDs, or optical media.
  • Bandwidth impact: Smaller file sizes mean fewer packets sent over networks, reducing transfer latency and bandwidth consumption.

🧠 Exam Technique

Always state the direct outcome. Don't simply say "it makes the file smaller" — explain why that matters (e.g., saves disk space or transmits faster across a network).

❌ Common Errors

  • Vague claims such as "makes the computer run faster" or "increases CPU speed".
  • Confusing compression with encryption (e.g., "to keep the data secret or safe from hackers").
Mark Allocation: 1 mark for AO1 (Understanding) — either reduced storage needs OR quicker network transfer times.
QUESTION 05.2 • 1 MARK

Applying Run Length Encoding (RLE)

State the RLE for the data shown in Figure 2:

B B G G G R R R R W

📐 Step-by-Step Run Identification

  1. Count consecutive B 's: 2 × B
  2. Count consecutive G 's: 3 × G
  3. Count consecutive R 's: 4 × R
  4. Count consecutive W 's: 1 × W

✅ Acceptable Representations

  • Standard tuple notation: (B, 2) (G, 3) (R, 4) (W, 1)
  • Reversed pair format: (2, B) (3, G) (4, R) (1, W)
  • Simplified delimiter-free: B2 G3 R4 W1 or 2B 3G 4R 1W

🧠 Marking Insights

  • Consistency is key: If you write frequency first ( 2, B ), you must do that for all runs.
  • Punctuation ignored: Examiners ignore brackets, commas, or spaces as long as the run count and character pairs are clearly associated.

❌ Common Errors

  • Omitting singleton runs: Writing just W instead of (W, 1) or 1W . The algorithm must record a run length for every sequence.
  • Swapping order mid-sequence: Mixing formats like B2 3G R4 1W forfeits the mark.
  • Miscounting repeating runs: Misreading 4 R 's as 3 or 5.
Mark Allocation: 1 mark for AO2 (Application) — all four runs correctly identified with consistent value/frequency pairings.

Topics

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

Question and mark scheme from the AQA A-Level Computer Science examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.