Mean Length of Utterance in Morphemes (MLU-M)
MLU-M is the average number of morphemes per utterance and remains the single most-used index of early grammatical development in English.
What MLU-M measures
Mean Length of Utterance in morphemes is calculated by dividing the total number of morphemes produced in a language sample by the total number of intelligible utterances. Brown introduced the metric in 1973 as a proxy for grammatical maturity in children up to roughly age 4, and it is now reported in almost every published language sample analysis. MLU counts bound morphemes such as plural -s, past tense -ed, and progressive -ing as separate units, which makes it more sensitive than a simple word count to early syntactic growth.
Formula
MLU-M = Σ morphemes ÷ number of complete and intelligible utterancesNormative ranges and benchmarks
- Age 2;0 — mean 1.9 morphemes (range 1.4 – 2.4)
- Age 3;0 — mean 3.2 morphemes (range 2.5 – 4.0)
- Age 4;0 — mean 4.4 morphemes (range 3.6 – 5.3)
- Age 5;0 — mean 5.2 morphemes (range 4.5 – 6.0)
- Above age 5 MLU-M flattens and loses sensitivity — switch to subordination index and DSS
Normative bands are central estimates drawn from the cited literature. Individual variation is wide — always cross-reference against the source paper and your assessment's own manual before quoting a cut-score in a report.
Clinical use
MLU-M is the single most common quantitative marker in a pediatric language report because it is fast to compute, highly reliable between raters, and has decades of published norms. In clinic it carries two jobs: first, as a developmental snapshot against Brown stages, and second, as a sensitivity check for the rest of the language sample — a child whose MLU is two standard deviations below age expectation virtually always has other markers of language impairment to investigate. Its ceiling hits hard after age 4;6, however; by first grade it is barely sensitive to clinically meaningful differences. For school-age children, most clinicians rely on MLU only as a screening number and pivot to clause density or DSS for the diagnostic work.
“MLU is the blood pressure of a preschool language sample. It is quick, boring, and almost useless in isolation — but if it is two SD below age expectation you already know the appointment is going to run long.”
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Free tools that compute MLU-M
MLU Calculator
Paste a language sample and get Mean Length of Utterance in morphemes and words, total utterances, total morphemes, and the matching Brown's stage. Implements Brown (1973) morpheme counting rules and runs entirely in your browser.
Open toolBrown's Stages Lookup
Interactive reference for Brown's (1973) five stages of grammatical development. Look up a stage by child age or MLU, see the MLU range, acquired morphemes, example utterances, and clinical milestones for each stage.
Open toolLanguage Sample Worksheet
Free printable and fillable language sample analysis worksheet for speech-language pathologists. Five columns (utterance #, transcription, morpheme count, grammatical Y/N, notes), configurable row count up to 100 utterances, browser print produces a clean PDF, and an inline running summary tracks total utterances, total morphemes, and rolling MLU as you fill it in.
Open toolRelated LSA metrics
Mean Length of Utterance in Words (MLU-W)
MLU-W counts whole words rather than morphemes and is the practical fallback when morpheme tagging is not reliable.
DSSDevelopmental Sentence Score (DSS)
DSS assigns weighted point values to eight grammatical categories across a 50-utterance sample to produce a single age-sensitive score.
IPSynIndex of Productive Syntax (IPSyn)
IPSyn gives two points per structure type for 56 syntactic structures in a child's transcript, capturing the diversity of syntactic forms they actually produce.
References
- Brown, R. (1973). A First Language: The Early Stages. Harvard University Press.
- Rice, M. L., Smolik, F., Perpich, D., Thompson, T., Rytting, N., & Blossom, M. (2010). Mean length of utterance levels in 6-month intervals for children 3 to 9 years with and without language impairments. JSLHR, 53(2), 333–349.
- Pavelko, S. L., & Owens, R. E. (2017). Sampling Utterances and Grammatical Analysis Revised (SUGAR): New normative values for language sample analysis measures. LSHSS, 48(3), 197–215.