File compression — re-encodes data into fewer bytes
File compression reduces a file's size — the number of bytes needed to store or send it — by re-encoding its data more efficiently. It does not delete the content the file represents; it changes how that content is stored, so the file uses less space and less bandwidth to transmit. A compressed file is normally extracted (for a .zip) or unarchived/unpacked (for a .rar) before it can be used again, so compression and decompression form a pair.
.zip and .rar — the two named compression formats
The specification names two file compression formats. .zip is a widely-used compressed archive: one or more files are compressed into a single, smaller .zip. .rar (Roshal Archive) is also used for data compression and can act as a data container that holds several files inside one archive. Both formats reduce the total size of the file(s) involved, AND let separate files be stored, moved or sent as one compressed file rather than many.
Two reasons to reduce size — storage and transmission
The specification names two separate reasons to reduce a file's size — a full answer gives both. For storage: a device (hard disk, memory stick, cloud account) has a limited capacity, so compressing files means more of them fit in the same space. For transmission: sending a file over a network takes time and bandwidth, so a smaller file transmits faster and is more likely to fit a system's attachment or transfer size limit.
Drawn from real examiner reports.
Compression vs image resolution/colour depth
File compression (.zip, .rar) re-encodes any file's data to shrink it. Reducing an image's resolution or colour depth is a different, image-specific way to cut file size that belongs to Section 12 (Images) — it is not file compression. Calling resolution/colour-depth changes "compression", or vice versa, blurs a boundary the syllabus keeps distinct.
Percentage: compressed ÷ original, then ×100
For "the compressed size as a percentage of the original", divide the compressed size by the original size, then multiply by 100 — e.g. 2 MB from 8 MB gives 2 ÷ 8 × 100 = 25%. Dividing the original by the compressed size (8 ÷ 2 = 400%) inverts the ratio, and forgetting the ×100 leaves a bare fraction.
Compression ≠ resizing/cropping
Asked how a file's size could be reduced, candidates often describe cropping an image or reducing its height and width, instead of naming compression. Resizing/cropping changes what the file contains; compression re-encodes the same content into fewer bytes. A "reduce the file size" question is asking for compression, not resizing.
Describe a format with TWO elements, not one
Describing .zip or .rar scores for two distinct elements, not for restating one point. "It's a zipped file" or "it makes files smaller" repeats a single idea and earns one mark only. Give two separate facts — for .rar: (1) it is used for data compression, AND (2) it is a data container holding multiple files in one archive.
Give both reasons, not a vague word
"Explain the need to reduce file sizes" wants both named reasons, each tied to a specific outcome. Naming only storage OR only transmission, or a bare label like "storage" or "speed", misses marks. State that more files fit the available capacity (storage) AND that files upload/download faster or fit a size limit (transmission).
Compression re-encodes — it does not delete
Compression changes how a file's data is stored, not what it represents. Answers implying it "deletes" or "throws away" part of the file are wrong — the full content returns once the file is decompressed. Compression re-encodes the same information into fewer bytes; nothing the file represents is lost when it is extracted again.
Say "compression", not "resizing"
If a question asks how a file's size can be reduced — and does not specifically ask about an image's resolution or dimensions — the expected answer is file compression. Resizing or cropping changes the content; compression re-encodes the same content into fewer bytes.
Two distinct facts for a format description
Describing .zip or .rar needs two separate facts — e.g. "used for data compression" AND "a container that holds multiple files" — not one point reworded. Check the two elements say genuinely different things before you move on.
Both reasons, each with its outcome
"Explain the need to reduce file size" wants both reasons: for storage (more files fit the available capacity) and for transmission (faster upload/download, or fits a size limit). State each with its specific outcome, not a single vague word.
A "discuss" answer needs a limitation
When the command word is discuss (or evaluate), weigh a limitation as well as the benefit — e.g. compression takes processing time, and the recipient must decompress the file before using it. A one-sided "benefits only" answer caps the marks a discuss question can reach.
File compression — the process of reducing the size of a file (the number of bytes needed to store or send it) by re-encoding its data more efficiently, without deleting the content the file represents.
.zip — a widely-used compressed archive file format that combines and compresses one or more files into a single, smaller .zip file.
Full notes, flashcards, Q&A and the topic quiz for every premium subject.
Premium plans are US$8.99/month or US$49.99/year — first month free.
Studying with a parent's blessing? Show them this.
