compares the frequency in a specific text type (part of corpus) to the whole corpus or compares frequencies in different text types (parts of corpus) even if they are not the same size. Thus the user can see whether the search word(s) is typical only for a specific text type (e.g. in newspapers only) but not in the rest of the corpus.
The number is relative frequency of the query result
divided by relative size of the particular text type
. It can be interpreted as “how much more/less often is the result of the query in this text type in comparison to the whole corpus”. Higher frequency means higher value, bigger text type size means lower value.
E.g. The word 'test' has 2000 hits in the corpus. 400 of them are in the text type “Spoken” and this text type represents 10 % of the corpus. Then the Relative Text Type frequency
will be (400 / 2000) / 0.1 = 200 % and it means 'test' is twice as common in “Spoken” than in the whole corpus.
see also Statistics in Sketch Engine