Lossy vs Lossless: Side-By-Side — 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.

This comparison comes up regularly in exam questions worth 3 to 4 marks.

Feature	Lossless	Lossy
Is any data lost?	No	Yes (permanently)
Can the original be reconstructed?	Yes, perfectly	No
Typical size reduction	Modest (1.5× to 4×)	Large (10× or more)
Effect on quality	None; quality identical to the original	Quality is reduced
Best for	Text documents, source code, archives, anywhere every bit matters	Photos, music, video, anywhere small size matters and minor quality loss is acceptable
Examples	PNG, FLAC, ZIP, GIF, RLE, PDF text	JPEG, MP3, MPEG/H.264, AAC

If the file is a text document or spreadsheet, choose lossless: a single missing word changes the meaning.
If the file is source code or a database backup, choose lossless: a single missing bit can crash the program.
If the file is a photograph for a website, choose lossy: a 10× smaller image that looks almost identical is a much better user experience.
If the file is a music track to stream over a mobile network, choose lossy: most users cannot hear the difference between a 320 kbps MP3 and the studio master, and the smaller size saves bandwidth.