Edexcel GCSE Computer Science (1CP2) Exam Questions

• 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 Tebibyte (1)
• The only correct answer is C. The correct answer is tebibyte.
• A is not correct because a gibibyte is 1024 mebibytes
• B is not correct because a mebibyte is 1024 kibibytes
• D is not correct because a terabyte is 1000 gigabytes

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 each of:

• 5 × 1024^3 / 5 × 1024 × 1024 × 1024 / 5 × 2^30, for the number of bytes (1)
• A fully correct expression with × 8 to convert bytes to bits, in any order, e.g. 5 × 1024^3 × 8, 8 × 5 × 2^30 (1)

Accept 5 × 1 073 741 824 × 8.

Accept 5 × 2^33 or 42 949 672 960 as a fully correct expression (2 marks).

Do not accept 5 × 1000^3 × 8, which uses decimal multiples.

Do not accept ÷ 8, which converts bits to bytes.

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.

Award up to two marks for a linked explanation such as:

• No data/instructions are removed (1) so the program still works/runs correctly after decompression (1)
• The original file can be recreated exactly (1) so all the code/instructions are present when the program is run (1)
• Lossy compression would permanently remove some data (1) so the program would not execute correctly/could crash (1)
• Every instruction is needed for the program to run (1) so the code must not be changed/lost (1)

For both marks, the expansion must follow/associate with the statement.

Allow 'no data/instructions are removed/lost' on its own as the point mark (1); the second mark needs the consequence for the program, e.g. it still works/runs correctly, or all the instructions are present.

Award credit for a contrast with lossy compression only when the loss of data is linked to the program failing/not running correctly.

Do not accept 'lossless is better quality'/keeps the quality without saying that data/instructions are not lost.

Do not accept 'it makes the file smaller'/'downloads faster'/'transmits faster' as a reason for using lossless, as both types of compression reduce file size.

Award up to two marks for a linked explanation such as:

• Lossy compression permanently removes some data (1) whereas lossless compression does not remove any data (1)
• With lossy compression the original file cannot be recreated exactly (1) but with lossless compression it can (1)
• Lossy compression can produce a smaller file/a higher compression ratio (1) whereas lossless compression keeps all of the data so compresses less (1)

For both marks, the expansion must follow/associate with the statement.

Allow 'removes data' for 'permanently removes data'.

A correct statement about only one type of compression, with no contrast, earns one mark.

Do not accept a reversed statement such as 'lossless compression loses data and lossy compression does not'.

Do not accept 'lossy makes the file smaller' on its own, because lossless compression also reduces the file size; the answer must say that data is removed.

Do not accept 'lossy is worse quality' on its own without saying data is removed.

Indicative content

How lossy compression makes the files smaller

• Some of the data in the file is permanently removed, so the file that is stored is smaller.
• The data that is removed is the data that a person is least likely to notice, such as sounds outside the range of human hearing or small differences in colour in a photograph.
• Removing data means a much higher compression ratio is possible than with lossless compression.

The effect on the photographs and audio

• The original file cannot be reproduced exactly when it is decompressed.
• Photographs may show artefacts, such as blocks or noise where there are contrasting colours, so they are lower quality.
• Some of the sound is lost, so parts of the audio guide may be less clear to listen to.
• If the company later wants to use the photographs at a larger size, it cannot recover the lost detail.

The benefits to the company and its visitors

• The smaller images load faster, so each page of the website appears more quickly for a visitor.
• The audio can be streamed, so a visitor can play the guide without waiting for the whole file to download.
• Less bandwidth is used by the web server, so it can cope with more visitors at the same time.
• Less storage space is needed on the server for the photographs and audio files.

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 the effect on the files and the benefits.

Do not accept the idea that lossy compression keeps all of the original data.