Simple maths is a method for identifying keywords of one corpus vs another. It includes a variable which allows the user to turn the focus either on higher, or lower frequency words.
Generally, a higher value (100, 1000, …) of Simple maths focuses on higher-frequency words (more common words), whereas a lower value (1, 0.1, …) of Simple maths focusses on low-frequency (more rare words).
The statistic used for keywords is a variation on “word W is so-and-so times more frequent in corpus X than corpus Y”. The keyness score of a word is calculated according to the following formula:
Your focus corpus (BNC): 112,289,776 tokens
Frequency of the lemma (shard) in the corpus: 35
Selected reference corpus (ukWaC): 1,559,716,979 tokens
Frequency of the lemma (shard) in the corpus: 263
For more details see:
Adam Kilgarriff. Simple maths for keywords. In Proceedings of Corpus Linguistics Conference CL2009, Mahlberg, M., González-Díaz, V. & Smith, C. (eds.), University of Liverpool, UK, July 2009.
Statistic used in Sketch Engine (Chapter 5). Lexical Computing Ltd., 8 July 2015.