Years 9–10 Data & Information Definitions Worksheet: Data compression (Modules 6–16)
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Key terms
Write what each term means, in your own words.
1
redundancy
2
lossless compression
3
lossy compression
4
4A3B
5
3C2D = CCCDD
6
run-length encoding
7
RLE expansion
8
dictionary compression
9
variable-length coding
10
compression ratio = original size / compressed size
Answer Key
Years 9–10 Data & Information · Modules 6–16 · Student wording will vary for definitions
1redundancy — Repeated or predictable data, like a run of the same letter or colour, that can be stored in a shorter form without losing any information.
2lossless compression — Shrinks a file so the exact original data can be rebuilt perfectly, with nothing lost. ZIP and PNG use it. Needed for text and programs.
3lossy compression — Shrinks a file by permanently throwing away detail people are unlikely to notice, so the original can't be rebuilt exactly. JPEG and MP3 use it.
44A3B — Run-length encoding: each run of identical characters becomes a count then the character, so AAAABBB is written 4A3B. A single character is written 1X.
53C2D = CCCDD — Decoding run-length data: read each count then its character, and write that character that many times. 3C means CCC and 2D means DD, so 3C2D expands back to CCCDD.
6run-length encoding — Compress a black-and-white image by writing each row as counts of same-coloured pixels in a run, starting with white. Row 00000 111 00 becomes 5,3,2. Counts can rebuild the grid.
7RLE expansion — When data has few repeats, run-length encoding adds a count to every character, so the result can be longer than the original. It only saves space when the data has long runs.
8dictionary compression — Give each repeated word or phrase a short code, store the code table once, then replace every repeat with its code so the data takes up fewer characters.
9variable-length coding — Gives common characters short bit codes and rare ones longer codes, so a whole message uses fewer bits than if every character used the same fixed number of bits.
10compression ratio = original size / compressed size — Original size divided by compressed size. Percentage saved = (original - compressed) / original x 100. Example: 10 MB down to 2 MB is a ratio of 5 to 1, saving 80%.