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J277 1.2.5 Compression

Part 4 of 5 · J277 1.2.4-1.2.5 · Data representation

Lossy and lossless compression, taught as a decision about the file and the user rather than a pair of definitions.

Students will:

  • explain why compression may be needed
  • distinguish lossy from lossless compression
  • explain the effect of each on a file
  • recommend an appropriate type for a scenario
  • justify the recommendation from the file and the user

Inside: 4 text sections, 3 multiple-choice activities, 1 completion activity, 4 written answers. 13 marks. Suggested teaching time: 35–55 minutes. 1 diagram.

Includes a teacher guide with suggested sections, timings and common misconceptions.

Series: J277 1.2.4-1.2.5 · Data representation, part 4 of 5.

Shared by Coding PathwayVerified teacher

  • 12 cells
  • About 45 minutes
  • CC BY-SA 4.0
  • Shared 17 Aug 2026
  • Updated 27 Oct 2026

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

Compression

Compression aims to reduce file size. Smaller files need less storage and less data to transmit. They can download sooner on the same connection or fit an attachment limit.

This does not make a photograph's width and height smaller automatically.

Multiple choice1 mark

Why might a school compress a photograph before uploading it?

  • ATo guarantee a higher-resolution photograph
  • BTo reduce the amount of data that must be sent
  • CTo add more colours to each pixel
  • DTo make every file use one byte

1. Can the exact original return?

Lossless compression uses a more efficient representation while retaining the information needed to reconstruct the exact original. After decompression, the original data and quality are unchanged.

Lossy compression permanently removes some data. The exact original cannot be reconstructed. For a photograph or recording, the quality change may be acceptable or hard to notice.

Lossy methods can often achieve a greater size reduction, but the result depends on the data and settings. Lossless does not mean uncompressed; a file that is already compressed may offer little further reduction.

Two compression choices and reconstruction The same original can be compressed losslessly or lossily. Lossless retains enough data to restore the exact original. Lossy permanently removes some data so the exact original cannot be restored. Both aim to reduce file size. Original data Lossless Exact original can be restored No data lost Lossy Some data removed forever Exact original cannot return Both aim to reduce file size.
Fill in the blanks2 marks
The exact original can be restored after Answer 1 compression. Some data is permanently removed by Answer 2 compression.
  • lossless
  • lossy
Written answer2 marks

State the key difference between lossy and lossless compression when the file is decompressed.

Students type their answer here.

2. Use the file's purpose

For a program or important text file, every instruction or character may need to return exactly: lossless is appropriate.

For a small preview photograph, a slight quality change may be acceptable if a smaller file downloads faster: lossy may be appropriate.

A master copy kept for later editing or printing may need exact preservation. Keep the original or use lossless compression. Explain the requirement, rather than simply saying one type is always better.

Written answer2 marks

A school must send an important text report and recover every character exactly. Recommend a compression type and justify it.

Students type their answer here.

Written answer3 marks

A school website needs preview photographs to download quickly. A small loss of picture quality is acceptable. Recommend a compression type, explain its download benefit and identify its possible drawback.

Students type their answer here.

Multiple choice1 mark

A designer must preserve a master image exactly for future editing. Which choice fits this requirement?

  • ALossy, because the exact discarded data always returns
  • BLossless, because the exact original can be restored
  • CLossy, because it automatically adds detail
  • DLossless, because it always gives the smallest possible file

3. Keep the terms precise

Lossy compression is normally unsuitable for an important text or program file because missing data can corrupt it or change its meaning. This is different from making an image of some letters look blurred.

Lossless compression can reduce storage needs while allowing exact restoration. It cannot improve detail that was missing from the original.

Multiple choice1 mark

Why is lossy compression normally unsuitable for program code?

  • APermanent data loss could change or corrupt the instructions
  • BIt makes every line of code longer
  • CIt cannot change file size
  • DIt always makes the program run faster
Written answer1 mark

A losslessly compressed photograph is decompressed. What should be true of its data compared with the original?

Students type their answer here.