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Edexcel GCSE Computer Science (1CP2) Exam Questions

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3 questions · 5 marks

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1Short recall1 markCompression Swap Remove

A company backs up its files to an external hard drive.

Give one reason why the company compresses the files before backing them up.

[1 mark]
Mark scheme

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'.

Example answers and how they were marked
Full marks1/1

The compressed files take up less storage space, so more files can be backed up.

Why: The files take up less storage space (1).

No marks0/1

It reduces the amount of disk fragmentation.

Why: Near-miss: compression does not reduce disk fragmentation.

No marks0/1

It encrypts the files so they are safer if the drive is stolen.

Why: Misconception: compression does not encrypt files, and security is not accepted.

2Short recall2 marksCompression Swap Remove

Files can be compressed before they are stored.

Define the term lossless compression.

[2 marks]
Mark scheme

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.

Example answers and how they were marked
Full marks2/2

Lossless compression reduces the size of a file without permanently removing any data, so the original file can be restored exactly.

Why: Reduces the size of a file (1) and no data is lost, so the original can be restored exactly (1); maximum of two marks reached.

Some marks1/2

It makes a file smaller so it downloads more quickly.

Why: Reduces the size of a file (1); the faster download is ignored as a benefit, not part of the definition.

No marks0/2

It removes data that is not needed to make the file smaller.

Why: Describes lossy compression, which the do-not-accept line rules out, so the mention of making the file smaller is not credited.

3Describe2 marksCompression Swap Remove

Photographs are often saved as JPG files.

Describe how lossy compression reduces the size of a photograph.

[2 marks]
Mark scheme

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.

Example answers and how they were marked
Full marks2/2

It permanently removes some of the pixel data that the human eye is unlikely to notice, such as small colour changes.

Why: Linked point: data removed permanently (1) and development: detail least noticeable (1).

Some marks1/2

Some of the data in the photograph is removed permanently.

Why: States permanent removal of data (1); no linked development is given (1).

No marks0/2

It finds repeated pixels and stores them once, so no data is lost.

Why: Describes lossless compression, not lossy; no credit.

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