Edexcel GCSE Computer Science (1CP2) Exam Questions

• B To reduce the time taken to transmit a file (1)
• The only correct answer is B. The correct answer is to reduce the time taken to transmit a file.
• A is not correct because compression reduces file size, which can reduce the number of packets sent
• C is not correct because lossy compression can reduce the quality of a file
• D is not correct because encryption, not compression, can make a file harder to open

• 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 8192 (1)
• The only correct answer is C. The correct answer is 8192.
• A is not correct because 1024 is the number of bytes in a kibibyte
• B is not correct because 8000 is the number of bits in 1000 bytes
• D is not correct because 8 388 608 is the number of bits in a mebibyte

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:

• 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.