Edexcel GCSE Computer Science (1CP2) Exam Questions

• C No data is lost and the original file can be recreated (1)
• The only correct answer is C. The correct answer is no data is lost and the original file can be recreated.
• A is not correct because encryption hides data and does not make the file smaller
• B is not correct because permanently removing data is lossy compression
• D is not correct because splitting a file into packets is part of transmission, not compression

• B Data is permanently removed to reduce the file size (1)
• The only correct answer is B. The correct answer is data is permanently removed to reduce the file size.
• A is not correct because the data removed by a lossy algorithm is not restored after transmission
• C is not correct because that describes lossless compression, where the original file is rebuilt exactly
• D is not correct because dictionary encoding is a lossless method that records repeated patterns once

• C Data is restored to its original format (1)
• The only correct answer is C. The correct answer is data is restored to its original format.
• A is not correct because encryption changes data so that it cannot be read without a key; it does not restore the data
• B is not correct because permanently removing data to reduce the file size is lossy compression, not decompression
• D is not correct because storing data using fewer bits is compression, not decompression

• C 8192 (1)
• The only correct answer is C. The correct answer is 8192.
• A is not correct because 1024 is the number of bytes in a kibibyte
• B is not correct because 8000 is the number of bits in 1000 bytes
• D is not correct because 8 388 608 is the number of bits in a mebibyte

Award one mark for:

• 2048 (1)
• 2 × 1024 (1)
• 2^11 (1)

Allow 2048 bytes.

Do not accept 2000, which is the number of bytes in two decimal kilobytes.

Do not accept 1024, which is the number of bytes in one kibibyte.

Do not accept 16, which is the number of bits in two bytes.

Award one mark for any of the following up to a maximum of two marks:

• Reduces/makes the size of a file smaller (1)
• No data is lost/removed (during compression) (1)
• The original file/data can be recreated/restored exactly (1)

Allow 'the compressed file takes up less storage space' for the first mark.

The second and third bullets are two ways of making the same point: award a maximum of one mark for them, so a response that says no data is lost and that the original file can be restored, but does not say the file is smaller, scores one mark.

Do not accept descriptions of lossy compression, e.g. 'it removes data that is not needed to make the file smaller'.

Ignore reference to download/transmission times, bandwidth and streaming, as these are benefits of compression and not part of the definition.

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.

Indicative content

Secondary storage

• Compressed lesson videos take up less space on the school's file server, so more recordings can be stored.
• The cloud backup service has a fixed amount of storage, so compressing the files means more of them can be backed up within it.
• Video files are very large, so without compression a much bigger hard disk would be needed to keep them all.

Sending data over a network

• A compressed video is split into fewer packets, so it takes less time for a student to download it at home.
• Less data is transferred, so the school's internet connection is not used up as quickly and other users are not slowed down.
• A smaller file can be sent as an email attachment, where there is a limit on the size of an attachment.
• Compressed video can be streamed, because the data can be sent as fast as it is played, which reduces buffering.

Drawbacks of compressing the files

• The file has to be compressed before it is stored and decompressed before it is played, which takes time and processing power.
• Lossy compression permanently removes data, so the quality of the lesson recording is reduced and it cannot be restored exactly.
• Lossless compression keeps all of the data, but it does not make the files as small as lossy compression does.

Levels

• Level 0 (0 marks): No rewardable content.
• Level 1 (1–2 marks): Basic, independent points are made, showing elements of understanding of key concepts/principles of computer science. (AO1) The discussion will contain basic information with little linkage between points made or application to the context. (AO2)
• Level 2 (3–4 marks): Demonstrates adequate understanding of key concepts/principles of computer science. (AO1) The discussion shows some linkages and lines of reasoning with some structure and application to the context. (AO2)
• Level 3 (5–6 marks): Demonstrates comprehensive understanding of key concepts/principles of computer science to support the discussion being presented. (AO1) The discussion is well developed, with sustained lines of reasoning that are coherent and logically structured, and which clearly apply to the context. (AO2)

Level 3 responses must cover both secondary storage and sending data over a network.

Do not accept the idea that compression makes the files more secure.