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TTR

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.

What TTR measures

Type-Token Ratio is the number of different word roots (types) divided by the total number of words (tokens) in a language sample. It was introduced by Templin in 1957 and has sat in nearly every undergraduate LSA textbook ever since. A TTR of 0.50 means half of the words in the sample are unique; 0.30 means only a third are. Conceptually it answers the question "how often does this child repeat the same words?" — a lower ratio means more repetition.

Formula

TTR = NDW ÷ TNW

Normative ranges and benchmarks

  • Age 3;0 — 0.45 (typical range 0.40 – 0.50)
  • Age 5;0 — 0.48 (typical range 0.43 – 0.53)
  • Age 8;0 — 0.50 (typical range 0.45 – 0.55)
  • TTR is only stable within a fixed sample size — never compare TTR across samples of different lengths
  • TTR below 0.35 in a school-age child almost always signals a lexical-retrieval or word-finding problem

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

TTR is the metric every LSA textbook teaches and the metric every practising clinician has to handle with care. Its main diagnostic weakness is that TTR falls as the sample grows, purely because a bigger sample accumulates more repetitions of high-frequency function words. That means two apparently good TTR values from samples of different lengths are not actually comparable. In reports I use TTR only alongside NDW at a fixed sample length and never as the only lexical index. Its use case is intra-child comparison on two same-length probes — story retell versus conversation, or before versus after a vocabulary intervention. For cross-child comparison the field has largely moved to MATTR and VOCD.

“TTR is a beginner’s trap. Every new SLP reaches for it, and every experienced SLP explains why we no longer report it without a matched sample length. If your report has a single TTR number in it, readers will rightfully ask "compared to what?"”
Use with matched sample length or not at all

Free tools that compute TTR

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 tool

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 tool

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 tool

Related LSA metrics

NDW

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.

MATTR

Moving-Average Type-Token Ratio (MATTR)

MATTR computes TTR across a sliding window and removes the sample-length confound that makes raw TTR unreliable.

VOCD / D

Vocabulary 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

  1. Templin, M. C. (1957). Certain Language Skills in Children. University of Minnesota Press.
  2. Richards, B. (1987). Type/token ratios: What do they really tell us? Journal of Child Language, 14(2), 201–209.
  3. McCarthy, P. M., & Jarvis, S. (2007). vocd: A theoretical and empirical evaluation. Language Testing, 24(4), 459–488.