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Year 10
OCR

Compression ratios

I can describe and calculate compression ratios.

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New
New
Year 10
OCR

Compression ratios

I can describe and calculate compression ratios.

Download all resources
Share activities with pupils

These resources will be removed by end of Summer Term 2025.

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Lesson details

Key learning points

  1. The compression ratio is the ratio between the size of the original data and the size of the compressed data.
  2. A higher compression ratio usually reduces the file size more, but it can slow down performance or lower quality.
  3. When selecting compression algorithms data quality and processing speed must be considered.
  4. Higher compression ratios often require more processing power and time for both compression and decompression.

Keywords

  • Original size - the file size before any compression has been applied

  • Ratio - a comparison of the original file size to the compressed file size

  • Compressed size - the file size after compression has been applied

Common misconception

A higher compression ratio always means better quality.

A higher compression ratio actually means the file size has been reduced more. This often results in lower quality, especially with lossy compression.


To help you plan your year 10 computer science lesson on: Compression ratios, download all teaching resources for free and adapt to suit your pupils' needs...

Use real file size examples and simple maths to help students understand compression ratios. Show how a file shrinks (e.g., 10 MB to 2 MB = 5:1), and use visuals to connect the ratio to changes in quality.
Teacher tip

Equipment

Licence

This content is © Oak National Academy Limited (2025), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Prior knowledge starter quiz

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6 Questions

Q1.
What is the main characteristic of lossy data compression?
It increases file size.
It does not lose any data.
Correct answer: It permanently removes some data.
It improves quality.
Q2.
Which of the following is true about lossless compression?
Correct answer: It reduces file size without losing data.
It removes some data permanently.
It increases file quality.
It works only on images.
Q3.
What is the effect of lossy compression on quality?
The quality improves.
The quality remains the same.
Correct answer: The quality is reduced.
The quality becomes perfect.
Q4.
How does lossy compression affect file size?
It increases file size.
Correct answer: It reduces file size significantly.
It keeps file size the same.
It doubles the file size.
Q5.
What type of compression does the JPEG format use?
lossless
Correct answer: lossy
uncompressed
hybrid
Q6.
What type of compression does the PNG format use?
uncompressed
hybrid
lossy
Correct answer: lossless

Assessment exit quiz

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6 Questions

Q1.
Match each term to its impact.
Correct Answer:lossy compression,reduces quality and file size

reduces quality and file size

Correct Answer:lossless compression,preserves all data

preserves all data

Correct Answer:compression ratio,affects file size and performance

affects file size and performance

Q2.
What is a potential downside of a higher compression ratio?
improved performance
increased quality
Correct answer: slower processing speed
reduced file size
Q3.
Which factors should be considered when choosing a compression algorithm?
colour depth
file extension and location
file name
Correct answer: data quality and processing speed
Q4.
What happens to a file size with a higher compression ratio?
It increases.
It stays the same.
Correct answer: It reduces more.
It doubles.
Q5.
Match each term to its definition.
Correct Answer:original size,the file size before compression

the file size before compression

Correct Answer:compressed size,the file size after compression

the file size after compression

Correct Answer:ratio,a comparison of original and compressed sizes

a comparison of original and compressed sizes

Correct Answer:processing speed,time taken for compression or decompression

time taken for compression or decompression

Q6.
Does a higher compression ratio always mean better quality?
Yes, because it improves the resolution of the file.
Correct answer: No, it means the file size has been reduced more, often lowering quality.
Yes, because it makes files easier to process.
No, it has no impact on file quality.