Edexcel GCSE Computer Science (1CP2) Exam Questions

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

• Reduces the size of a file (1)
• Permanently removes/loses some data (1)
• The original file cannot be recreated/restored exactly (1)

Allow 'some data is deleted/discarded and cannot be recovered' for the second mark.

Do not award the first mark where the response also states that no data is lost.

Do not accept 'removes repeated data' (this describes lossless compression).

Award one mark for any of the following:

• More files/data can be stored (in the same storage space) (1)
• The files take up less storage space (1)
• The backup is quicker/faster to complete (1)
• Less data needs to be transferred to the backup device (1)

Do not accept 'makes the files more secure' or 'encrypts the files'.

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

• Repeated words/patterns are found (1) and stored once in a dictionary with a code (1)
• Each repeated word/pattern is replaced by a shorter code (1) so fewer bits are needed to store the text (1)
• No data is removed (1) so the original text can be recreated exactly (1)
• The compressed file is smaller (1) but all the original characters can still be restored (1)

Do not accept answers that say data is permanently removed or quality is reduced.

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.