Data Compression

Compression reduces the size of a file. Smaller files take up less storage and move faster across networks. There are two broad approaches: lossless, which rebuilds the original exactly, and lossy, which throws away some detail to save more space.

This topic is part of Big Idea 2: Data. You will learn the trade-off between size and quality and when each kind of compression is appropriate.

Lossless compression finds repeated patterns and stores them more efficiently, then restores the file exactly when uncompressed. Nothing is lost. This is essential for text, code, and any file that must stay identical.

Lossy compression permanently removes detail the human eye or ear is unlikely to notice. It usually achieves much smaller files, which is why it is used for photos, music, and video, but the original cannot be perfectly recovered.

The compression ratio describes how much smaller the file became. Higher compression saves more space but, for lossy methods, can reduce quality. The right choice depends entirely on the file's purpose.

Visual Comparison

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Lossless restores the file exactly; lossy produces a smaller file with some detail removed.

Follow each row left to right. In the top row the restored file matches the original perfectly. In the bottom row the file is smaller, but the dashed outline and faded bars show detail that is gone for good. On the exam, match the method to whether exact recovery is required.

• Compression: reducing the number of bits needed to store data.
• Lossless: compression that can restore the original data exactly.
• Lossy: compression that permanently discards some detail to save more space.
• Compression ratio: how much smaller the compressed file is compared to the original.
• Quality: how closely the result matches the original.
• Transmission speed: how quickly a file can be sent; smaller files travel faster.

Be ready to:

• Choose lossless or lossy for a given situation and justify it.
• Explain the trade-off between file size and quality.
• Recognize that compression affects storage and transmission speed.
• Identify that lossless is required when data must be preserved exactly.

The exam values clear reasoning about purpose. The correct answer connects the compression type to what the file is used for.

A school website has large image files that load slowly. Which compression should they use?

For photos in the news gallery: lossy compression is usually best. Visitors will not notice slight quality loss, and the much smaller files make pages load faster, improving the experience.

For a downloadable logo or official document scan that must stay crisp: lossless compression is better, because the image needs to remain exact when reused or printed.

The reasoning is about purpose, not size alone. Faster loading favors lossy for casual photos, while accuracy favors lossless for files that must be preserved. A good answer states the purpose, names the method, and explains the trade-off.

• Thinking compression always loses quality. Lossless compression keeps everything.
• Thinking smaller is always better. Over-compressing can ruin important detail.
• Using lossy when exact data is required. Text and code must stay identical, so lossy is wrong there.
• Forgetting the effect on transfer. Compression also changes upload and download time.
• Confusing compression with encryption. Compression shrinks data; encryption protects it. They are different goals.

When a question asks which compression to use, write your answer as "Use [method] because the file needs to [purpose]." Naming the purpose, such as "must be restored exactly" or "needs to load quickly," shows the reasoning that earns full marks.

Before data can be stored or compressed, it has to be collected. In the next topic you will explore where data comes from, how its quality and bias affect conclusions, and why privacy and consent matter when gathering information.