relative frequency, frequency per million

(also called freq/mill in the interface)

a number of occurrences (hits) of an item per million, also called i.p.m. (instances per million). It is used to compare frequencies between corpora of different sizes.

number of hits : corpus size in millions of tokens = frequency per million

The frequency per million is always related to the whole corpus or subcorpus, not to a text type. Restricting the query to one or more text types will affect the number of hits but the frequency per million will stay calculated using the number of tokens in the whole (sub)corpus.

To relate the frequency per million to one or more text types, create a subcorpus from the text type(s) and restrict the query to this subcorpus.

Example

Looking up the frequency of the word helps in the British National Corpus (112,181,015 tokens), first in the spoken Text type and then in the spoken subcorpus will produce these results.

SUBCORPUS SELECTED none none spoken
11,787,138 tokens
TEXT TYPE SELECTED none spoken none
HITS 3,116 302 302
FREQUENCY PER MILLION 27.75
in relation to the number of tokens in the whole corpus
2.69
in relation to the number of tokens in the whole corpus
25.62
in relation to the subcorpus size
POSSIBLE INTERPRETATION helps appears 27.75 times per million words in BNC ‘spoken’ helps appears 2.69 times per million in BNC helps appears 25.62 times per million in the spoken part of BNC

see also

Statistics in Sketch Engine
Average Reduced Frequency