Lossless Compression — Data Storage and Compression | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 4% of your exam marks.

stable

Rare

Stable4%

File size calculations and lossless vs lossy compression are regular 3 to 4 mark questions.

Lossless compression reduces file size without removing any data. The original file can be perfectly reconstructed when the file is decompressed.

Key features:

No data is lost. Every bit of the original can be recovered.
The reduction in size is usually modest (typical lossless compression gives ratios from 1.5:1 to 4:1, depending on the content).
It is the right choice when every bit matters: text documents, source code, spreadsheets, medical images, archival masters.

How it works: lossless methods look for patterns, redundancy and repetition in the data and replace them with a more compact code that can later be reversed. There are many algorithms, but the one named in the IGCSE syllabus is run length encoding (RLE).

Common lossless formats: PNG images, FLAC audio, ZIP archives, GIF images, PDF documents (which use lossless compression internally).