OCR J277 Mark Scheme - Compression

OCR	GCSE (9-1) Computer Science Mark Scheme J277/01: Unit 1.2 Compression

Question	Answer	Marks	Guidance
1a	It reduces the file size / takes up less storage space (1). It takes less time to transmit / downloads faster / uses less bandwidth (1).	2	AO1 (Knowledge) Examiner Note: Do not accept "It is faster" without qualification (e.g. faster to send). Do not accept "It clears up memory".
1b	Row 1: Lossless Row 2: Lossy Row 3: Lossy	3	AO1 (Knowledge) 1 mark per correct tick. If multiple ticks in one row, award 0 for that row.
2a	Lossless	1	AO2 (Application)
2b	Lossy compression removes data permanently (1). If code/text is lost, the program will not run / will contain errors / become unreadable (1).	2	AO2 (Application) Must link the removal of data to the consequence for a code file (crashing/errors). Accept: "The computer needs the exact code to understand the instructions."
3a	It creates a significantly smaller file size (than lossless) (1). Which means the webpage will load faster for the user (1).	2	AO2 (Application) Key concept: Trade-off. The file is much smaller, which is prioritized over perfect quality for web use.
3b	The quality of the image will reduce (1). It might look pixelated / blurry / contain artefacts (1).	2	AO1 (Knowledge) Accept "Data is lost permanently" for 1 mark if not used in Q2b, but the question asks for the effect on the image, so quality/pixelation is the preferred answer.
4	Lossy: Permanently removes data so the file cannot be restored to its original state (1). Lossless: Retains all data / looks for patterns so the file can be restored to its exact original state (1). Comparison: Lossy results in lower quality but smaller size, whereas Lossless keeps original quality but larger size (1).	3	AO1 (Knowledge) Max 3 marks. Award 1 mark for defining Lossy (permanent loss). Award 1 mark for defining Lossless (perfect restoration). Award 1 mark for a valid comparison of the result (quality vs size).