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.
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
Practice questions for Compression
Which statement correctly describes lossy compression?
Explain how run-length encoding (RLE) works to compress data.