Data can be compressed using lossy or lossless compression.
Describe one difference between lossy compression and lossless compression.
Mark scheme
Award up to two marks for a linked description such as:
- Lossy compression permanently removes/discards some of the data (1) whereas lossless compression keeps all of the original data so the file can be restored/rebuilt exactly (1)
- Lossy compression cannot reproduce/recreate the original file exactly (1) whereas lossless compression can decompress the file back to its original form (1)
- Lossy compression can make a file smaller than lossless compression (1) because more of the original data is removed/discarded (1)
- Lossy compression is used for images/sound/video (1) whereas lossless compression is used where all the original data must be kept, such as program files (1)
Allow one mark for a correct statement about one type of compression only, e.g. 'lossy compression permanently removes some data', with no comparison or development.
Do not accept 'lossy compression loses quality' on its own unless it states that data/information is removed or lost permanently.
Do not accept 'lossy is better' or 'lossless is safer' with no described difference.
Do not award a smaller file size on its own; it must be linked to data being removed or to no data being lost.
Example answers and how they were marked
Lossy compression permanently removes some of the data, whereas lossless compression keeps all of the original data so the file can be restored exactly.
Why: Linked difference: lossy permanently removes data (1) and lossless keeps all data so the file is restored exactly (1).
Lossy compression permanently removes some of the data.
Why: A correct statement about one type only (1), with no comparison to lossless or development, so the second mark is not earned.
Lossless compression removes the data that is not needed so the file is smaller, but lossy compression keeps every bit of the data.
Why: The two types are the wrong way round; no creditworthy point, so no marks.