Number of Different Words (NDW)
NDW counts unique word roots across a fixed sample length and is the most stable lexical-diversity measure for school-age children.
What NDW measures
Number of Different Words is the count of distinct word roots in a language sample of standardised length. Word roots are counted once regardless of inflection — "run", "ran", "running", and "runs" all collapse to a single entry. Because NDW rises as a child produces more total words, it is always reported for a fixed sample size — most commonly NDW in 50 or 100 utterances in SALT or 25 utterances in SUGAR.
Formula
NDW = count of unique word roots, reported at a fixed sample length (e.g., 50 utterances)Normative ranges and benchmarks
- Age 4;0, 50 utterances — 95 – 130 NDW (SALT)
- Age 6;0, 50 utterances — 130 – 165 NDW
- Age 8;0, 50 utterances — 155 – 195 NDW
- Age 12;0, 50 utterances — 195 – 240 NDW
- NDW is a ratio-free lexical marker — it does not require dividing by total words
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
NDW is what most clinicians reach for when they want a lexical-diversity number that is stable across short samples. Because it avoids the ratio-based pitfalls of TTR, it remains interpretable across typically developing and language-impaired children and it discriminates well into early adolescence. Its weakness is sample-length sensitivity — a 30-utterance sample and a 70-utterance sample from the same child produce different NDW values. For that reason, clinicians should only compare NDW to norms that specify the exact sample length used to derive them. In reports we usually pair NDW with TTR and with MATTR to paint a fuller picture of lexical growth.
“If a third-grader produces 120 NDW in 50 utterances the vocabulary is fine. If the same child produces 60, it is time to ask the classroom teacher what curriculum words are missing from the conversation — not to order a norm-referenced vocabulary test.”
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Free tools that compute NDW
Lexical Diversity Calculator
Paste a language sample and get type-token ratio (TTR), number of different words in the first 100 tokens (NDW-100, Miller 1981), and NDW per 50 utterances (NDW-50, SUGAR). Implements the standard SALT/SUGAR tokenisation rules and runs entirely in your browser.
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 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
Type-Token Ratio (TTR)
TTR divides unique word roots by total words to index lexical diversity — quick to compute but highly sensitive to sample length.
MATTRMoving-Average Type-Token Ratio (MATTR)
MATTR computes TTR across a sliding window and removes the sample-length confound that makes raw TTR unreliable.
VOCD / DVocabulary Diversity (VOCD / D-measure)
VOCD fits a mathematical curve to TTR values at many sub-sample sizes to produce a single length-invariant diversity score known as D.
References
- Templin, M. C. (1957). Certain Language Skills in Children. University of Minnesota Press.
- Miller, J. F. (1991). Quantifying productive language disorders. In Research on Child Language Disorders. Pro-Ed.
- Watkins, R. V., Kelly, D. J., Harbers, H. M., & Hollis, W. (1995). Measuring children's lexical diversity: Differentiating typical and impaired language learners. JSLHR, 38(6), 1349–1355.