The need for compression, the difference between lossy and lossless compression, and how run-length encoding compresses data.

A Unit 1 compare question. Lossy compression makes a file smaller by permanently removing some data, usually detail that people are less likely to notice, so the original cannot be perfectly recovered (1 mark); it is used for things like streaming music or photos where a small loss of quality is acceptable in return for much smaller files (1 mark). Lossless compression makes a file smaller without losing any data, so the original can be reconstructed exactly (1 mark); it is used where every bit matters, such as compressing a text document, a spreadsheet or program code (1 mark). Markers reward the permanent loss versus exact recovery distinction and a sensible example of each. A common error is to say lossless makes files smaller by deleting data, which describes lossy.

A Unit 1 run-length encoding question. Run-length encoding records each run as the value and the number of times it repeats. The row is five W, then six B, then four W, so it compresses to $5\text{W}\,6\text{B}\,4\text{W}$ (or pairs such as $(5,\text{W})(6,\text{B})(4,\text{W})$) (2 marks for the correct encoding). It saves space because the data contains long runs of identical values, so storing the value once with a count is shorter than listing every value (1 mark). Markers reward the value-and-count pairs and the reason about repeated runs. A common error is to record each character separately, which does not compress, or to lose the order of the runs.