T-units and C-units
T-units and C-units are the two competing ways of segmenting a transcript into analyzable chunks for syntactic analysis.
What T-units / C-units measures
A T-unit ("minimal terminable unit") was defined by Hunt in 1965 as one independent clause plus any subordinate clauses attached to it — the smallest stretch of language that can be terminated as a grammatical sentence. A C-unit ("communication unit") was defined by Loban in 1976 as an independent clause with its modifiers that can also stand as a complete conversational turn, even if it is not a full grammatical sentence. The choice between them is methodological: T-units are stricter and better suited to written or academic-register samples, C-units are more permissive and capture the realities of conversational speech.
Formula
Count T-units or C-units by applying Hunt (1965) or Loban (1976) rules to the transcriptNormative ranges and benchmarks
- For conversational samples in typically developing school-age children, C-units and T-units usually agree within 5%
- For highly elliptical conversation (yes/no answers, single-phrase replies), C-units include utterances T-units exclude
- For narrative retells and expository monologues, T-units and C-units converge nearly perfectly
- The choice of unit must be reported in the methods section — it changes every downstream metric
- Subordination index and clause density should always use the same unit they were originally normed on
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
Most school-based clinicians report C-units because they work on conversational samples, and C-units do not punish a child for producing a grammatically incomplete but pragmatically adequate turn. Research reports more often use T-units because they are stricter and more reproducible. The critical clinical rule is never to mix them in a single report — if you compute MLU-M on T-units but look up norms that are based on C-units, the difference is large enough to move a child across a clinical cut-score. Document your segmentation choice and stay consistent across the child's longitudinal record.
“The fastest way to invalidate a language sample is to mix T-units on Monday and C-units on Friday. Pick one at the start of the year and write it into every report template you touch.”
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Free tools that compute T-units / C-units
Language 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 toolMLU 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 toolIPSyn Calculator
Free Index of Productive Syntax (IPSyn) calculator for speech-language pathologists. Score the 56 Scarborough (1990) grammatical items across the Noun Phrase, Verb Phrase, Question/Negation, and Sentence Structure subscales from a 100-utterance language sample. Live IPSyn total, per-subscale subtotals, sample-size guard, mobile-friendly, client-side, no sign-up.
Open toolRelated LSA metrics
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.
SISubordination Index (Clause Density)
The Subordination Index, or clause density, divides total clauses by total T-units and indexes syntactic elaboration in school-age and adolescent samples.
PCUPercent Complex Utterances (PCU)
PCU is the share of a child’s utterances that contain two or more clauses — the clearest single marker of emerging syntactic complexity.
References
- Hunt, K. W. (1965). Grammatical Structures Written at Three Grade Levels. NCTE.
- Loban, W. (1976). Language development: Kindergarten through grade twelve. NCTE.
- Scott, C. M., & Stokes, S. L. (1995). Measures of syntax in school-age children and adolescents. LSHSS, 26(4), 309–319.