Edexcel GCSE Computer Science (1CP2) Exam Questions

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

• Some data is removed permanently (1) choosing detail that people are least likely to notice (1)
• Small changes in colour/shade are discarded (1) so fewer different values need to be stored (1)
• Similar pixels are stored as one value (1) so there are fewer values in the file (1)
• Fine detail is removed (1) so fewer bits are needed to store each area (1)

Do not accept answers that say repeated patterns are stored once / no data is lost.

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

• The file size is reduced (1) so the documents take less time to transmit/send (1)
• The compressed files are smaller (1) so they are more likely to be within the email attachment size limit (1)
• Less data is sent (1) so less bandwidth/data allowance is used (1)
• Fewer packets are needed (1) so the documents are received more quickly (1)

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

Do not accept 'it saves storage space' on its own without linking to sending the documents.

Do not accept 'it makes the file smaller' as both the point and the reason.

Indicative content

Why lossless compression is suitable

• No data is removed, so the original file can be restored exactly when it is decompressed.
• Source code must be reproduced exactly, because a single missing or changed character would stop the program working when it is run.
• A text document must keep every character, so that the meaning of the document is not changed.

How lossless compression makes a file smaller

• Run length encoding stores a repeated value once, together with a count of how many times it appears, so long runs of the same character are held in less space.
• Dictionary encoding finds patterns or words that are repeated in the text, stores each one once in a dictionary and replaces every occurrence with a short code.
• The more repetition there is in a file, the greater the reduction in size that can be achieved.

Benefits to the company

• The compressed files take up less space on the file server, so more programs and documents can be stored.
• There is less data to transfer, so backing the files up to another location finishes sooner.
• The company does not need to keep a separate uncompressed copy as well, because the compressed file can be restored exactly, which saves more storage space.
• The files must be decompressed before they are used, which takes time each time a file is opened.

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 why lossless compression is suitable and how it makes a file smaller.

Do not accept the idea that lossless compression removes data that is not needed.