Pearson Edexcel · GCSE · Computer Science
Course version: 1CP2 · Version from Sept 2020
Understand the need for data compression and methods of compressing data…
Full specification wording
Data storage and compression: 2.3.2 understand the need for data compression and methods of compressing data (lossless, lossy)
Compression reduces the number of bits used to store or transmit data.
Checked against the Pearson Edexcel 1CP2 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Compression reduces the number of bits used to store or transmit data. It is needed to save storage space, reduce transmission time and lower bandwidth use, and to make files quicker to upload or download. Lossless compression reduces file size while allowing the original data to be reconstructed exactly, so it suits text, spreadsheets, source code and archives; run-length encoding and dictionary-based methods are typical. Lossy compression permanently discards some detail that humans are less likely to notice, giving much smaller files for images, audio and video, but the original cannot be restored exactly. Choosing a method depends on whether exact recovery matters: a text document needs lossless, while a photograph for a web page may accept lossy. Both methods reduce file size, but only lossless preserves every original bit.
What earns the marks
- Compression reduces file size, which saves storage space and reduces the time and bandwidth needed to transmit data.
- Lossless compression allows the original data to be reconstructed exactly, so it is used where no detail may be lost, such as text or program files.
- Lossy compression permanently removes some data, producing smaller files for images, audio and video where a slight loss of quality is acceptable.
- The choice between lossless and lossy depends on whether exact reconstruction of the original data is required.
Common mistakes and how to put them right
Saying lossy compression can be reversed to the original.
Put it right: Lossy compression permanently loses detail, while lossless restores the original exactly.
Believing compression always saves storage but never affects quality.
Put it right: Lossy compression trades quality for a smaller size.
Thinking lossless compression works by deleting repeated data.
Put it right: Lossless methods encode data more efficiently, for example by replacing repeated patterns with shorter codes, so nothing is lost.
Exam tips
- Link each method to a suitable file type, such as lossless for text and lossy for a photograph, to show understanding of the trade-off.
- Use the phrase 'reconstructed exactly' for lossless and 'permanently discarded' for lossy to keep the distinction precise.
- When asked why compression is needed, give both a storage reason and a transmission reason rather than only one.
Try the lesson
The start of the Data storage and compression: 2.3.2 understand the need for data compression and methods of compressing data (lossless, lossy) lesson, made from checked teaching notes for this specification point.
Have a go first
A school website stores large photographs for its news pages on a server with limited storage space, and many visitors download the pages over slow connections. What is the main reason for compressing these image files before they are stored?
Listen
Compression reduces the number of bits used to store or transmit data. It is needed to save storage space, reduce transmission time and lower bandwidth use, and to make files quicker to upload or download. Lossless compression reduces file size while allowing the original data to be reconstructed exactly, so it suits text, spreadsheets, source code and archives; run-length encoding and dictionary-based methods are typical. Lossy compression permanently discards some detail that humans are less likely to notice, giving much smaller files for images, audio and video, but the original cannot be restored exactly. Choosing a method depends on whether exact recovery matters: a text document needs lossless, while a photograph for a web page may accept lossy. Both methods reduce file size, but only lossless preserves every original bit. A strong answer earns marks for these points. Compression reduces file size, which saves storage space and reduces the time and bandwidth needed to transmit data. Lossless compression allows the original data to be reconstructed exactly, so it is used where no detail may be lost, such as text or program files. Lossy compression permanently removes some data, producing smaller files for images, audio and video where a slight loss of quality is acceptable. The choice between lossless and lossy depends on whether exact reconstruction of the original data is required. A common slip: Saying lossy compression can be reversed to the original: the error is claiming the discarded data can be recovered. Instead, lossy compression permanently loses detail, while lossless restores the original exactly. Exam tip: Link each method to a suitable file type, such as lossless for text and lossy for a photograph, to show understanding of the trade-off.
Key words
Say what each word means before you turn the card.
Need for compression
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Lossless compression and exact reconstruction
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Lossy compression and permanent data loss
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Matching compression method to file type
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See every definition
- Need for compression
- Compression reduces the number of bits used to store or transmit data, saving storage space and reducing transmission time and bandwidth use.
- Lossless compression and exact reconstruction
- Reduces file size while allowing the original data to be reconstructed exactly, so it suits text, spreadsheets, source code and archives.
- Lossy compression and permanent data loss
- Permanently discards some detail that humans are less likely to notice, giving much smaller files for images, audio and video, but the original cannot be restored exactly.
- Matching compression method to file type
- Choosing a method depends on whether exact recovery matters, so a text document needs lossless while a photograph for a web page may accept lossy.
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