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1CP2-P-9.5 Lossless and lossy compression

Part 5 of 6 · 1CP2-P-9 · Bitmap, sound and compression

Compression reduces the number of bits used to store or transmit data. Lossless compression allows the original to be reconstructed exactly. Lossy compression permanently removes or approximates information to achieve a smaller representation.

Students will:

  • distinguish lossless from lossy compression
  • use exact reconstruction as a selection criterion
  • explain why compression can reduce transfer time
  • recommend an approach for a stated context

Inside: 7 explanation cells, 1 fill-in-the-blanks cell, 2 multiple-choice questions and 3 written answers. 12 marks, about 45 minutes.

Series: 1CP2-P-9 · Bitmap, sound and compression, part 5 of 6.

Shared by Coding PathwayVerified teacher

  • 13 cells
  • About 45 minutes
  • CC BY-SA 4.0
  • Shared 17 Aug 2026

Preview

The whole resource, exactly as a class sees it. Answers and marking are held back.

Lossless and lossy compression

Compression reduces the number of bits used to store or transmit data. Lossless compression allows the original to be reconstructed exactly. Lossy compression permanently removes or approximates information to achieve a smaller representation.

1. Build the representation

Lossless and lossy reconstructionoriginal101100101losslessno information removedlossysome detail removed101100101exact reconstruction101100001approximation differs

Lossless is required where every symbol or bit matters, such as program code or text records. Lossy may suit images or sound when some quality reduction is acceptable. Compression is not backup, encryption or guaranteed error protection.

Fill in the blanks3 marks
gap 1 compression reconstructs the exact original. gap 2 compression permanently removes or approximates detail. Both aim to reduce file gap 3.
  • lossless
  • lossy
  • size
Multiple choice1 mark

Which file most clearly requires exact reconstruction?

  • AProgram source code
  • BA preview photograph where small quality loss is acceptable
  • CA streamed music preview
  • DA decorative background image

2. Worked method

A school archives assessment records with lossless compression because every character must be recovered. It may use lossy compression for website photographs if the visual reduction is acceptable and smaller downloads matter.

Written answer2 marks

Explain why lossy compression is unsuitable for executable program data.

Link removed information to correctness.

Students type their answer here.

Written answer2 marks

Explain one benefit of compressing files before network transmission.

Link size to transferred bits and a consequence.

Students type their answer here.

3. Apply and check

Write the formula or reasoning step before substituting values. Keep the representation, the measurement and its units distinct.

Written answer3 marks

Recommend lossless or lossy compression for medical notes stored as text, and justify the choice.

Use exactness and consequence.

Students type their answer here.

Multiple choice1 mark

Which statement is accurate?

  • ALossless cannot reduce file size
  • BDecompressing lossy data always restores the original
  • CCompression creates a separate recovery copy
  • DLossless trades some redundancy for smaller storage while preserving exact data

Examination method

Show relevant working, label units and answer the command word. A number without its meaning or unit may not demonstrate the required method.

Repeated editing matters

If a lossy file is decoded, edited and saved lossily again, further information may be removed at each save. Keep an exact or lossless master when future editing must preserve as much original information as possible.

Route forward

The checkpoint will combine bitmap, sound and compression without introducing named compression algorithms.