Storage units each hold 1024 of the previous
A bit (binary digit) is the smallest unit — a single 0 or 1. A nibble is 4 bits; a byte is 8 bits. Above the byte, each unit holds 1024 of the previous one (the binary convention Cambridge requires — 1024, not 1000): kibibyte (KiB) = 1024 bytes, mebibyte (MiB) = 1024 KiB, gibibyte (GiB) = 1024 MiB, then tebibyte, pebibyte and exbibyte. In calculations you must use 1024 between adjacent units and give the answer in the unit the question asks for.
File size: image and sound formulas
Image file size (bits) = number of pixels × colour depth (resolution = width × height; colour depth = bits per pixel). Sound file size (bits) = sample rate × sample resolution × length (sample rate = samples per second in Hz; sample resolution = bits per sample). To convert: bits ÷ 8 = bytes, then ÷ 1024 = KiB, then ÷ 1024 = MiB. Always finish in the unit specified, and always divide using 1024.
Lossless keeps all data; lossy removes it
Lossless compression reduces size with no permanent data loss — fully recoverable; the named example is run-length encoding (RLE). Used when no data may be lost: text, spreadsheets, programs. Lossy compression removes data permanently (not recoverable), so quality drops — e.g. lowering image resolution/colour depth or sound sample rate/resolution. Used when a small quality loss is acceptable: streaming, web photos. Compression saves storage, bandwidth and transmission time.
Drawn from real examiner reports.
Dividing by 1000 instead of 1024
For IGCSE storage units you always divide by 1024 (not 1000) between adjacent units — kibibyte, mebibyte and so on. Using 1000 gives the wrong figure: 480000 bytes is 468.75 KiB (÷1024), not 480 KiB (÷1000). And never forget the ÷8 step from bits to bytes first.
Describing lossy when asked for lossless
A common error is mixing up the two methods. Lossless reduces file size without permanent loss of data — the original is fully recoverable. Lossy reduces file size by permanently removing data, so quality is reduced and the data cannot be recovered. Describing one when the question asks for the other scores nothing; give the definition for the method actually asked.
June 2023 examiner report: candidates describing lossless compression of a text file gave answers more reflective of lossy compression.
Describing image compression for a text file
When a question is about compressing a text file, candidates wrongly describe compressing an image — reducing colour depth or resolution. You cannot reduce the colour depth of text. For a text file the lossless method to describe is run-length encoding (replacing repeated characters with a character and a count), or a dictionary of repeated words.
June 2023 examiner report: candidates gave confused descriptions relating to images rather than text.
Makes it smaller distinguishes neither method
A vague answer such as 'it makes the file smaller' earns zero when a question asks about a compression method, because it is true of both lossy and lossless and so distinguishes neither. Always add the distinguishing element: lossless keeps all the data (fully recoverable); lossy permanently removes data (quality reduced).
Giving a format instead of a compression type
When a question asks for the type of compression, the answer is lossy or lossless — not a file format such as MP3, JPEG or ZIP. A format is an example, not a type. Read whether the question wants the type or an example and answer accordingly; naming MP3 for a 'type of compression' question earns no mark.
June 2023 examiner report: some candidates gave an example of compression rather than a type — for example, MP3.
Sample rate vs sample resolution
Sample rate and sample resolution are different: sample rate is the number of samples taken per second (Hz); sample resolution is the number of bits used to store each sample. Candidates often define one when asked for the other. Increasing either improves sound quality but increases file size, and reducing either is a form of lossy compression.
November 2023 examiner report: the most common incorrect answers described the sample rate instead of the sample resolution.
Work the file size in stages
Show each stage of the calculation — method marks apply even if the final figure is wrong. Find the size in bits (image = pixels × colour depth; sound = sample rate × sample resolution × length), then ÷8 for bytes, ÷1024 for each step up, stop at the requested unit.
Name the compression type, then justify
For 'which compression and why' questions, name the type (lossless or lossy), then justify by the data: lossless if no data may be lost (text, a program); lossy if a small quality drop is acceptable for a much smaller file (streamed music, web photos).
Use 1024, and never skip the divide by 8
Between byte-based units always divide by 1024, not 1000, and never forget the ÷8 from bits to bytes — leaving the answer in bits when the question asks for bytes or KiB is a common slip that loses marks.
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.
