The purpose of compression, the difference between lossy and lossless compression, and how to choose the appropriate type for a given scenario.

Lossy (up to 2 marks): some data is permanently removed to make the file much smaller, so the original cannot be fully restored; the loss is chosen to be hard for people to notice. Suitable use: streaming or storing music (MP3) or photos (JPEG), where a small quality drop is acceptable.

Lossless (up to 2 marks): no data is lost, so the original can be reconstructed exactly, usually with a smaller reduction in size. Suitable use: text documents, spreadsheets or program files (ZIP, PNG), where every bit must be recovered.

Markers reward the "permanently removed, cannot restore" versus "no loss, can restore exactly" contrast, plus a sensible use for each.

Run-length encoding (up to 2 marks): it replaces runs of the same repeated value with a single value and a count of how many times it repeats, so a long run of identical data is stored much more compactly. For example a row of 20 identical white pixels is stored as "white, 20" rather than listing all 20.

Works well (1 mark): on data with long runs of repetition, such as simple images with large areas of one colour (icons, logos). It is lossless, so the original is fully restored.

Markers reward the value-plus-count idea and a data type with lots of repetition.