Speakability is the child’s skill in producing meaningful utterances. Words of an utterance may be divided into prefixes, roots and suffixes – the smallest units of meaning called morphemes. The Mean Length of Utterance is the total number of morphemes divided by the total number of utterances. Usually, a sample of 100 utterances is taken for calculating the MLU. Graham Williamson’s Mean Length Of Utterance: http://www.sltinfo.com/mean-length-of-utterance/ is a very fine and comprehensive article on the subject.
The Expected Mean Length of Utterance (EMLU) is a simple formula that can diminish parental anxiety about a child’s speakability. If M is the age of the child from Months 18 to 60, then EMLU = (M – 5) / 10. For example, if the child’s age is 25 months, then EMLU is 2 morphemes per utterance. Parents should understand that some children may be fast or slow in gaining speakability skills. The EMLU formula just gives a ballpark figure. Parents may be happy if their children produce more morphemes than the formula indicates, but should not worry if they produce less. Sooner or later, children are bound to pick up their native language.