Compression: lossy and lossless – GCSE Computer Science exam questions with mark scheme

1Grade 1–33 marks

A student attaches a 30 MB video to an email, but the email service rejects attachments larger than 25 MB. The student compresses the video before sending it.

(a) State what is meant by compression. [1]

(b) Give two other reasons why files are commonly compressed. [2]

(a) Reducing the size of a file / number of bits used to store the data (allow 'making a file smaller')(1)
(b) Any one of: takes up less storage space / more files fit on the device; faster to download / upload / transmit; uses less bandwidth / data allowance; faster to stream (audio / video); do not accept 'to fit within an attachment / size limit' (this is the reason given in the scenario)(1)
(b) A second different reason from the list above (max 2 for part (b))(1)
Answer: (a) Reducing the size of a file. (b) e.g. less storage space needed; faster to transfer over a network.
Examiner tip: In (b) 'to make it smaller' just repeats the definition – say what the smaller size is useful for (storage, transfer speed, bandwidth).

2Grade 1–34 marks

Compression can be lossy or lossless.

For each file below, state which type of compression would be most suitable.

File	Type of compression
(a) A holiday photo being uploaded to a social media site
(b) The source code of a program being sent to another developer
(c) A hospital X-ray scan stored in a patient's records
(d) A music track being streamed to a mobile phone

[4]

(a) Lossy(1)
(b) Lossless(1)
(c) Lossless(1)
(d) Lossy(1)
Answer: (a) lossy (b) lossless (c) lossless (d) lossy
Examiner tip: Ask: would losing some data be a problem? Code and medical images must be exact (lossless); photos and streamed music can tolerate small losses (lossy).

3Grade 4–55 marks

A 4.8 MB photograph is compressed using lossy compression. The compressed file is 1.2 MB.

(a) Calculate the percentage by which the file size has been reduced. Show your working. [2]

(b) Explain why lossy compression is appropriate for this photograph. [2]

(a) Reduction = 4.8 − 1.2 = 3.6 (MB) or 3.6 ÷ 4.8 (seen)(1)
(a) 75 (%) (allow 75% for correct answer with no working; also allow 'compressed to 25% of original' if clearly stated)(1)
(b) Lossy permanently removes some data / detail from the image …(1)
(b) … that the human eye is unlikely to notice / the quality loss is acceptable for a photograph, so a much smaller file is produced(1)
(c) Any one of: loss of image quality / detail; original image cannot be restored / recovered from the compressed file; artefacts / blockiness may appear; cannot be used where an exact copy is needed(1)
Answer: (a) 75% (b) removes data the eye will not notice, giving a much smaller file at acceptable quality (c) e.g. the original cannot be recovered
Examiner tip: Percentage reduction is (change ÷ original) × 100 – divide by the original 4.8 MB, not the new 1.2 MB.

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4Grade 4–53 marks

A software company distributes its program as a compressed download.

(a) Explain why lossless compression must be used for the program file. [2]

(b) State one disadvantage of lossless compression compared with lossy compression. [1]

(a) Lossless compression allows the original file to be rebuilt exactly / no data is lost when the file is decompressed(1)
(a) If any data / instructions were removed, the program would not run correctly / would be corrupted / would crash(1)
(b) The file size is not reduced as much / the compressed file is larger than it would be with lossy compression (so it takes more storage space and longer to transfer); do not accept 'quality is lost'(1)
Answer: (a) Every bit of the program must be restored exactly or it will not run. (b) e.g. gives a smaller reduction in file size than lossy.
Examiner tip: The second mark in (a) needs the consequence for the program – say what would go wrong if data were missing.

5Grade 4–55 marks

The bar chart shows the file size of the same photograph saved in four different ways.

A – original uncompressed file
B – lossless compression
C – lossy compression, high-quality setting
D – lossy compression, low-quality setting

(a) Using the chart, calculate the percentage reduction in file size from A to D. [2]

(b) Describe one difference the user would notice between image B and image D when viewed on screen. [1]

(a) (6.0 − 0.3) ÷ 6.0 (× 100) or 5.7 ÷ 6.0 seen(1)
(a) 95 (%)(1)
(b) D will have lower / poorer image quality (e.g. less detail, blurring, blocky areas / artefacts, fewer colours) than B; allow 'B looks identical to the original, D does not'(1)
(c) Lossless (B) keeps all of the original data / only removes redundancy, so the original can be restored exactly …(1)
(c) … whereas lossy (C) permanently removes some data, so it can reduce the size further(1)
Answer: (a) 95% (b) D looks lower quality / blocky; B looks identical to the original (c) B keeps all the data so can only remove redundancy; C discards data so goes smaller.
Examiner tip: Read the bar values carefully – the reduction is from the original (A), so divide by 6.0 MB.

6Grade 6–76 marks

A video-streaming company stores thousands of films and streams them to customers over the Internet.

Discuss whether the company should use lossy or lossless compression for the films it streams. In your answer you should consider the advantages and disadvantages of each type of compression. [6]

Lossy: much smaller files → less storage needed for thousands of films(1)
Lossy: smaller files need less bandwidth / stream with less buffering / work on slower connections / use less of customer's data allowance(1)
Lossy: some picture / sound quality is permanently lost; may show artefacts on large screens; original cannot be recovered(1)
Lossless: exact original quality is kept / no loss of data on decompression(1)
Lossless: files remain much larger → more storage, more bandwidth, more buffering, higher cost(1)
Lossless: compression ratio depends on the data; film data has little redundancy so savings are small(1)
Justified conclusion, e.g. lossy is better for streaming because the quality loss is barely noticeable while the saving in bandwidth and storage is large; lossless could be kept for master copies(1)

Level	Marks	What it looks like
Level 3	5–6	Thorough knowledge of both lossy and lossless compression, applied clearly to the streaming scenario, with a reasoned and justified conclusion.
Level 2	3–4	Reasonable knowledge of both types, with some application to streaming; conclusion present but may be only partly justified.
Level 1	1–2	Basic points about one or both types of compression, little or no application to the scenario; no real conclusion.
Level 0	0	No creditworthy response.

Answer: Lossy is the better choice for streaming: far smaller files reduce storage and bandwidth and avoid buffering, and the small quality loss is acceptable to viewers; lossless would keep perfect quality but files would be too large to stream smoothly. A balanced answer may suggest lossless master copies with lossy streaming versions.
Examiner tip: To reach the top level you must cover both types, link each point to the streaming context (bandwidth, buffering, storage) and finish with a justified recommendation.

7Grade 6–75 marks

A 60 MB file is to be downloaded over a connection with a speed of 8 Mbps (megabits per second).

(a) Calculate the time taken to download the uncompressed file. Show your working. [2]

(b) Before it is downloaded, the file is compressed so that its size is reduced by 70%. Calculate the time saved by downloading the compressed file instead. [2]

(a) 60 MB = 480 Mb (× 8) or 8 Mbps = 1 MB/s seen(1)
(a) 60 (seconds)(1)
(b) Compressed size = 18 MB (or 144 Mb) or compressed download time = 18 s(1)
(b) 42 (seconds) saved (allow follow-through from (a))(1)
(c) The file must be decompressed before it can be used, which takes (processing) time; allow 'time taken to compress the file first'(1)
Answer: (a) 60 s (b) 42 s saved (compressed file 18 MB downloads in 18 s) (c) the file still has to be decompressed before it can be used
Examiner tip: Watch the units – the file is in megaBYTES and the speed is in megaBITS per second, so multiply the file size by 8 (or divide the speed by 8).

8Grade 4–53 marks

A student says:

“I compressed my photo with lossy compression to make it smaller. When I decompress it, I will get my original photo back.”

(a) Explain why the student is wrong. [2]

(b) State which type of compression would allow the original file to be restored exactly. [1]

(a) Lossy compression permanently removes / discards data from the file(1)
(a) so the removed data cannot be recovered when the file is decompressed / the decompressed photo is lower quality than the original(1)
(b) Lossless(1)
Answer: (a) Lossy compression permanently deletes some data, so decompressing cannot bring it back. (b) Lossless.
Examiner tip: The key word is permanently – lossy compression removes data that is gone for good, not just hidden.

9Grade 6–75 marks

A company backs up its 2 TB file server every night to an external drive. Lossless compression reduces the backup to 60% of its original size.

(a) Calculate the size of the compressed backup in gigabytes. [2]

(b) Explain why lossy compression would not be suitable for the backup. [2]

(a) 2 TB = 2000 GB or 0.6 × 2 (TB) = 1.2 TB seen(1)
(a) 1200 (GB) (allow 1228.8 GB if 1024 conversion used)(1)
(b) The backup contains many types of file (documents, databases, programs) that must be restored exactly / identical to the originals(1)
(b) Lossy compression would permanently remove data, so restored files would be corrupted / incomplete / unusable(1)
(c) The processor must carry out extra work / run the compression algorithm on all the data before it is written (allow 'compression takes processing time')(1)
Answer: (a) 1200 GB (b) backups must restore files exactly; lossy would permanently remove data and corrupt restored files (c) compressing the data takes extra processing time
Examiner tip: In (a) convert TB to GB (× 1000) and give the answer in the unit asked for – 1.2 TB on its own scores only the first mark.

10Grade 4–54 marks

A music app streams songs to users' phones over mobile networks. The songs are stored using lossy compression.

(a) Explain one reason why the music is compressed before streaming. [2]

(b) Describe the effect that lossy compression has on the sound file. [2]

(a) Compressed files contain fewer bits / are smaller …(1)
(a) … so they stream with less buffering / need less bandwidth / use less of the user's mobile data allowance / download faster; allow 'more songs can be cached / stored on the phone'(1)
(b) Some of the sound data is permanently removed (e.g. frequencies humans cannot hear / very quiet sounds)(1)
(b) giving a (slightly) lower-quality sound but a much smaller file; allow 'original quality cannot be recovered'(1)
Answer: (a) Smaller files need less bandwidth so songs play without buffering and use less mobile data. (b) Data the listener is unlikely to notice is permanently removed, lowering quality slightly but greatly reducing file size.
Examiner tip: 'Explain' needs a reason and its consequence – link 'smaller file' to a benefit such as less buffering or less data used.

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