Compare lossy and lossless compression
Distinguish permanent data removal from reversible compression and select a method for a stated use.
Section 1 · Data representation
Exam scope and assessment notes
This sequence follows Cambridge IGCSE Computer Science 0984, 2026–2028 syllabus Version 4. It covers subject content sections 1.1–1.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding. Cambridge states that 0984 has the same 2026–2028 subject content as 0478, with 9–1 grading. In June and November, Cambridge lists 0984 in Administrative zone 3 and for centres in Oman and the UAE. Confirm availability for your centre and exam series with Cambridge International. The separately published 2029–2031 syllabus changes to Python 3 only and removes pseudocode; this 2026–2028 route does not cover those changes.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Can a lossless method restore the exact original data?
What you will learn
- Explain that lossy compression permanently removes data
- Explain that lossless compression preserves exact reconstruction
- Justify a compression choice from a file's purpose
Lossy compression trades detail for size
Lossy compression permanently removes some data, for example by reducing image resolution or colour depth, or reducing sound sample rate or resolution. The original cannot be reconstructed exactly from the compressed file.
Apply and check the rule
Lossless compression reduces size without permanently losing information. The exact original data can be restored. Repeated patterns can be represented more compactly using run-length encoding (RLE).
Pause and explain
Which file is the strongest candidate for lossless compression?
Worked example
A medical scan must remain unchanged for later review. Which type of compression is appropriate?
Show the worked solution
- The scan must be recoverable exactly.
- Lossy compression permanently removes detail.
- Lossless compression can reduce size without permanent data loss.
Answer Lossless compression, because it preserves the exact scan data for later reconstruction.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A photograph is compressed by permanently removing subtle colour information. Is this lossy or lossless?
Give me a hint
Ask whether the original pixel values can be reconstructed exactly.
Compare my reasoning
- Some colour data is permanently removed.
- The exact original cannot be recovered.
- That is lossy compression.
Lossy compression
Look for these in your work
- Permanent removal recognized
- Exact recovery ruled out
- Correct method named
A text file is encoded more compactly and then restored character-for-character. Which compression type is being used?
Give me a hint
The defining clue is exact restoration.
Compare my reasoning
- The original characters are all recovered.
- No information has been permanently removed.
- That is lossless compression.
Lossless compression
Look for these in your work
- Exact recovery recognized
- No permanent loss stated
- Correct method named
A photographer needs a small image for a web preview but must retain a master copy for printing. Recommend a compression approach for each file and explain the trade-off.
Give me a hint
The preview and master have different purposes.
Compare my reasoning
- Use lossy compression for the preview if a small quality reduction is acceptable.
- Keep the print master lossless or uncompressed so exact source detail remains available.
- This balances transfer size with future editing and print quality.
A lossy preview can reduce download size, while the master should preserve exact data through lossless storage or remain uncompressed. The preview's quality reduction is the trade-off.
Look for these in your work
- Preview method justified
- Master method justified
- Quality/size trade-off stated
Common mistakes
- Calling a compressed file lossless merely because it still looks similar
- Assuming that every lossless method makes every file smaller
What can you explain now?
Which method permanently removes some data?
Compare with the explanation
Lossy compression
Lossy compression reduces size by discarding data that cannot be restored exactly.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What is the defining difference between lossy and lossless compression?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.