Developmental Patterns and Discriminant Power of MLU and LLU in Korean Children Aged 2–5 Years

Objectives: This study aimed to describe age-related developmental patterns of Mean Length of Utterance (MLU) and Longest Length of Utterance (LLU) in typically developing Korean children and to examine their discriminant validity across adjacent age groups.Methods: A total of 240 typically developing children aged 2–5 years were divided into eight 6-month age groups (N=30 per group). Spontaneous language samples were collected through semi-structured conversation procedures, and 50 utterances per child were analyzed. MLU and LLU were calculated at the eojeol-, word-, and morpheme-level units. Two-way ANOVAs were used to examine the effects of age and sex, while one-sample ttests compared the present results with those of Kim (1997). Simple regression analyses evaluated the explanatory power of chronological age, and ROC analyses tested the discriminant ability between adjacent age groups.Results: All MLU and LLU measures showed significant age-related increases, with the most pronounced growth observed between late age 2 and around age 4. Girls consistently produced longer utterances than boys, although no significant age-by-sex interaction was found. Compared with Kim (1997), most age groups showed significantly higher MLU and LLU values. Chronological age significantly predicted all indices, and ROC analyses confirmed strong discriminant validity between adjacent age intervals.Conclusion: This study provides updated developmental norms and discriminant reference values for MLU and LLU in Korean children. The two indices complementarily reflect grammatical complexity (MLU) and expressive capacity (LLU), offering objective benchmarks for language development assessment and early detection of language delay.