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3.3 Data RepresentationExam Tips

Exam Tips - Compression

Part of Compression · GCSE GCSE Computer Science revision

This exam tips covers Exam Tips - Compression within Compression for GCSE Computer Science. Revise Compression in 3.3 Data Representation for GCSE Computer Science with 16 exam-style questions and 16 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 10 of 12 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.

Topic position

Section 10 of 12

Practice

16 questions

Recall

16 flashcards

Exam Tips - Compression

Most common exam questions:

  • "Difference between lossy and lossless?" → Lossless = perfect reconstruction; Lossy = permanent data loss
  • "When to use lossy?" → Photos, music, video - for human viewing/listening
  • "When to use lossless?" → Text, programs, medical images - need exact copy
  • "Examples of lossy?" → JPEG, MP3, MP4
  • "Examples of lossless?" → PNG, ZIP, FLAC
  • "How does RLE work?" → Replace repeated values with count + value

Key comparison facts:

  • Lossless: 30-70% compression, reversible, exact original
  • Lossy: 80-95% compression, irreversible, "good enough" quality
  • Trade-off: Quality vs file size - can't have both!

RLE exam questions:

  • Always show the pattern: AAAAAABBB → 6A3B
  • Count + Value format
  • Works best when there are LONG runs of repeated data
  • Can make files LARGER if no repetition

Common mistakes to avoid:

  • Saying "lossy is always better" - NO! Depends on use case
  • Confusing lossless with lossy file formats (JPEG is lossy, PNG is lossless)
  • Forgetting that lossy is PERMANENT - you can't "uncompress" to perfect quality
  • Not explaining WHY you'd use one over the other

Keep building this topic

Read this section alongside the surrounding pages in Compression. That gives you the full topic sequence instead of a single isolated revision point.

Real-World Example: Photo SharingQuick Reference - CompressionThe Travel Packing ProblemDeep Dive: Why Compression Matters

Practice Questions for Compression

Which statement correctly describes lossy compression?

  • A. The original file can be perfectly restored after decompression.
  • B. Data is permanently removed and the original cannot be exactly recreated.
  • C. The compressed file is always the same size as the original.
  • D. No data is removed during the compression process.
1 markfoundation

Explain how run-length encoding (RLE) works to compress data.

3 marksstandard

Quick Recall Flashcards

16 questions on Compression — practise free

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Quick Reference - Compression