Mean Length of Utterance and the Acquisition of Irish

One of the most widely used indices of language development is a measure of utterance length in morphemes (MLUm). This study examines the applicability of MLUm to the acquisition of Irish. MLUm was calculated for data from Cian, aged 1;11–3;0. Even when an attempt was made to ‘assume the maximum’ by counting all possible morphemes, the correlation between a morpheme MLU (MLUm) and a word count MLU (MLUw) was very high (0·99). This points to MLUw being as effective a measure of Irish development as MLUm, as well as being easier to apply and more reliable. MLUw was calculated for the two younger children in the study (Eibhlís 1;4–2;1 and Eoin 1;10–2;6). An examination of the relationship between the three children's MLUw values and their grammatical complexity as measured on ILARSP (the Irish adaptation of LARSP) indicates that MLUw is a useful preliminary index for early development in Irish. However, further data are necessary to check whether MLUw loses its predictive relationship with grammatical complexity after a certain point. The study emphasizes the caution necessary in applying MLU to languages whose acquisition has not hitherto been studied, and underlines the role of MLU as a preliminary measure, which must not be overinterpreted.