Community resourceWorksheet

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: 6 explanation cells, 4 multiple-choice questions, 1 fill-in-the-blanks cell and 4 written answers. 33 marks, about 60 minutes.

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

Shared by Coding PathwayVerified teacher

  • 15 cells
  • About 60 minutes
  • CC BY-SA 4.0
  • Shared 17 Aug 2026
  • Updated 9 Sept 2026

Preview

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

Data representation 4: compression

Text, image and sound files can occupy substantial storage space and take time to transmit. Compression reduces a file's size.

By the end of this worksheet, you should be able to:

  • explain why compression may be needed
  • distinguish lossy and lossless compression
  • explain the effects of each type on a file
  • compare their advantages and disadvantages
  • recommend an appropriate type for a scenario
  • justify the recommendation using the needs of the file and user

OCR does not require you to carry out or memorise specific compression algorithms.

Work through the explanation, checks and applications in order. Use the final challenge to connect several ideas.

1. Why files are compressed

Reducing file size can mean:

  • less storage space is required
  • more files fit on a device
  • less data needs to be transmitted
  • downloads or uploads can complete more quickly
  • less network bandwidth is used

Use precise language. “It is faster” is too vague. State that the smaller file can be transmitted, uploaded or downloaded more quickly. “It uses less space” should say storage space.

Multiple choice, several answers2 marks

Which two statements accurately describe benefits of compression? Select both.

  • AEvery compressed file has higher quality
  • BA smaller file requires less storage space
  • CCompression gives a storage device unlimited capacity
  • DA smaller file can require less data to be transmitted

2. Lossy and lossless compression

Lossy compression

  • Permanently removes some data.
  • The exact original file cannot be reconstructed.
  • It can produce a substantial reduction in file size.
  • Quality may be reduced.
  • It can be suitable for images and sound when a small quality loss is acceptable.

Lossless compression

  • Does not permanently lose the information needed to reconstruct the file.
  • The exact original can be restored after decompression.
  • It often produces a smaller reduction than lossy compression.
  • It is suitable when every part of the original must be preserved, such as text, program or data files.
Lossless and lossy compression compared Original data follows two routes. Lossless compression creates a smaller file that restores an exact copy. Lossy compression creates a smaller file by permanently removing some data and restores only an approximation. Originalall source data Losslessno data permanently lost Lossysome data permanently removed Exact restorationidentical to original Approximationremoved data cannot return
Fill in the blanks5 marks
label 1 compression permanently removes some data and cannot recreate the exact original. label 2 compression allows the exact original to be label 3. Lossy compression may reduce label 4, while both types aim to reduce label 5.
  • quality
  • restored
  • file size
  • resolution
  • lossless
  • lossy
  • sample rate
Multiple choice1 mark

Which statement describes lossless compression?

  • ASome data is permanently removed and the exact original cannot return
  • BThe image automatically gains more pixels
  • CThe exact original file can be reconstructed after decompression
  • DThe file is never made smaller
Written answer4 marks

Compare lossy and lossless compression. Give two developed differences.

Compare permanent data removal, reconstruction, file-size reduction or quality.

Students type their answer here.

3. Choose for the file and purpose

Compression choices depend on what the file contains and what the user needs.

  • Text or program file: lossless is normally appropriate because every character or instruction must be restored exactly.
  • Website photograph: lossy may be appropriate because a small visual change may be acceptable and a much smaller file can download more quickly.
  • Master recording or archival image: lossless may be chosen because preserving the exact original quality matters.

Do more than describe the compression type. Explain why it is suitable in this context.

Multiple choice, several answers3 marks

Which three choices are suitably justified? Select all that apply.

  • ALossless for program code because every instruction must be reconstructed exactly
  • BLossy for a legal text document because blurry letters are acceptable
  • CLossy for a website photograph when a small quality loss is acceptable and faster download matters
  • DLossless for every photograph because lossless always creates the smallest file
  • ELossless for a master audio archive because the exact original should be preserved
Written answer6 marks

A student must email a text report and several photographs. Recommend a compression type for each and justify both choices.

Treat the report and photographs separately. Explain whether exact reconstruction or a larger size reduction matters.

Students type their answer here.

4. Correct two common misconceptions

Misconception 1: “Lossy compression selects words and makes their letters blurry.”

Lossy compression is unsuitable for a text file because permanently losing data can corrupt the file or change its meaning. It does not simply apply a visual blur to selected letters.

Misconception 2: “Lossless means the file is not compressed.”

Lossless compression still reduces file size. The important difference is that the exact original can be reconstructed.

Multiple choice1 mark

Why is lossy compression normally unsuitable for an important text document?

  • AIt adds extra pages to the document
  • BPermanent data loss could corrupt or change the document, so the exact original is required
  • CIt always turns every character into an image
  • DIt prevents the file from becoming smaller
Written answer5 marks

A news website wants images to load quickly on mobile connections, but the photographs must remain clear enough to understand. Recommend a compression approach and explain the benefit and drawback.

State a type, connect smaller files to transmission, then recognise the possible quality effect.

Students type their answer here.

Written answer6 marks

Challenge: evaluate the statement, “Lossy is better because it makes files smaller.”

Consider file size, quality, exact reconstruction and at least two different file purposes before reaching a conclusion.

Students type their answer here.

Closed-book review

  1. Why are files compressed?
  2. What happens in lossy compression?
  3. What happens in lossless compression?
  4. Which type permits exact reconstruction?
  5. Which type may produce a greater size reduction?
  6. Why is lossless normally suitable for text?
  7. When might lossy be suitable for an image or sound file?
  8. Why is “it is faster” too vague?

Remember: the best compression type depends on the file and the user's priorities.