Subordination 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.
What SI measures
The Subordination Index — commonly called clause density — is the average number of clauses per T-unit in a language sample. Hunt introduced the concept in 1965 to study syntactic development in written composition, and Scott and Stokes later extended it to oral language. Values above 1.0 indicate that the child routinely produces more than one clause per T-unit, meaning subordination is present at some level. The metric is sensitive from about age 8 through adulthood, well beyond the MLU ceiling.
Formula
SI = total clauses ÷ total T-unitsNormative ranges and benchmarks
- Age 8;0 — SI ≈ 1.10 – 1.20
- Age 10;0 — SI ≈ 1.20 – 1.35
- Age 13;0 — SI ≈ 1.30 – 1.50
- Adult expository discourse — SI ≈ 1.40 – 1.70
- Children with language impairment typically cluster 0.10 – 0.20 below age expectation
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
SI is the metric of choice for adolescents on a language caseload, because by middle school almost every other LSA index has ceilinged out. Its diagnostic use is differential: a ninth-grader whose clause density is 1.05 is not producing enough subordination for academic language, and the therapy target writes itself. In practice, clinicians use the Nippold protocols — peer conflict explanations, favourite-game descriptions, expository monologue — because the elicitation context matters hugely at this age. A conversational sample with a shy teen will understate their clause density by a full standard deviation compared to a structured expository probe. The elicitation should match what the academic setting demands.
“If a 13-year-old is producing 1.0 clauses per T-unit in an expository sample, the whole academic year is quietly going to be about subordination. Write the goal now and save yourself the February parent conference.”
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Free tools that compute SI
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.
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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 toolDSS Calculator
Free Developmental Sentence Scoring (DSS) calculator for speech-language pathologists. Paste 50 sentences, tap weighted points across the eight Lee (1974) grammatical categories — Indefinite Pronouns, Personal Pronouns, Main Verbs, Secondary Verbs, Negatives, Conjunctions, Interrogative Reversals, Wh-Questions — plus the all-correct sentence point, and the calculator returns the live DSS with a per-category breakdown. Mobile-friendly tap-through grid, client-side, no sign-up.
Open toolRelated LSA metrics
Percent 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.
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
- Hunt, K. W. (1965). Grammatical Structures Written at Three Grade Levels. NCTE.
- Scott, C. M., & Stokes, S. L. (1995). Measures of syntax in school-age children and adolescents. LSHSS, 26(4), 309–319.
- Nippold, M. A. (2009). School-age children talk about chess: Does knowledge drive syntactic complexity? JSLHR, 52(4), 856–871.