Compression ratios
Lesson details
Learning outcome
I can describe and calculate compression ratios.
Key learning points
- The compression ratio is the ratio between the size of the original data and the size of the compressed data.
- A higher compression ratio usually reduces the file size more, but it can slow down performance or lower quality.
- When selecting compression algorithms data quality and processing speed must be considered.
- 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.
Teacher tip
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.
Licence
Lesson video
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Prior knowledge starter quiz
6 Questions
Q1.What is the main characteristic of lossy data compression?
Q2.Which of the following is true about lossless compression?
Q3.What is the effect of lossy compression on quality?
Q4.How does lossy compression affect file size?
Q5.What type of compression does the JPEG format use?
Q6.What type of compression does the PNG format use?
Assessment exit quiz
6 Questions
Q1.Match each term to its impact.
Q2.What is a potential downside of a higher compression ratio?
Q3.Which factors should be considered when choosing a compression algorithm?
Q4.What happens to a file size with a higher compression ratio?
Q5.Match each term to its definition.
Q6.Does a higher compression ratio always mean better quality?
To help you plan your 10 computer science lesson on: Compression ratios, download all teaching resources for free and adapt to suit your pupils' needs...
To help you plan your 10 computer science lesson on: Compression ratios, download all teaching resources for free and adapt to suit your pupils' needs.
The starter quiz will activate and check your pupils' prior knowledge, with versions available both with and without answers in PDF format.
We use learning cycles to break down learning into key concepts or ideas linked to the learning outcome. Each learning cycle features explanations with checks for understanding and practice tasks with feedback. All of this is found in our slide decks, ready for you to download and edit. The practice tasks are also available as printable worksheets and some lessons have additional materials with extra material you might need for teaching the lesson.
The assessment exit quiz will test your pupils' understanding of the key learning points.
Our video is a tool for planning, showing how other teachers might teach the lesson, offering helpful tips, modelled explanations and inspiration for your own delivery in the classroom. Plus, you can set it as homework or revision for pupils and keep their learning on track by sharing an online pupil version of this lesson.
Explore more key stage 4 computer science lessons from the Data compression unit, dive into the full secondary computer science curriculum, or learn more about lesson planning.