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 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 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.

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

• The repeated phrase is stored once (1) along with a count of the number of times it occurs (1)
• The repeated phrase is replaced by a shorter code/reference (1) and the original phrase is held in a dictionary/lookup table (1)
• A shorter code is used in place of each repeated phrase (1) so fewer characters/bits are needed to store the file (1)
• Groups of repeated characters are replaced by a count and a single value (1) so the file stores less data (1)

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

Allow naming of a suitable method, e.g. run-length encoding or dictionary coding, in place of a description of that method.

Award the marks once only where the same method (run-length encoding) is described more than once.

Do not accept 'it makes the file smaller' on its own without a method of compression.

Do not accept answers that describe lossy compression, e.g. that data/detail is removed or lost permanently, because lossless compression keeps all of the original data.

Do not accept 'it removes the repeated words/duplicates' on its own, as this suggests data is lost; credit only where the answer says the phrase is stored once with a count or code.