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Speech Intelligibility, "I don't understand..."

Intelligibility is a measure of how easy it is to understand a speaker. For kids it has often been measured through questioning their parents using an intelligibility scale. This then led to the development of a model for intelligibility that has been used for years.

Great, right? Sure, until a recent study (Hustad,,2021) proved that it may all be a little off the mark. Instead of using parent-based measures, this newer research used unfamiliar adults to try to work out what was being said by kids. Recordings were played to the adults who then had to listen and write down what they thought the children had said. Intelligibility was then calculated based upon how many words the adults wrote that matched what the children had actually said.

The results of this research led to a rethinking of how we look at intelligibility in terms of speech, and language acquisition. In the past the rule was:

  • 50% of what kids say should be understood by unfamiliar listeners at 2 years old

  • 75% by 3 years old

  • 100% understood by 4 years old


(Coplan & Gleason, 1988)


Hustad and colleagues’ research (2021) demonstrated that development of speech intelligibility was more variable than previously thought, and that levels of intelligibility in typically developing kids was more along the lines of:


  • 50% of what kids say should be understood by unfamiliar listeners by 3 years old

  • 75% understood by 4 years

  • Above 90% understood by 5 years


The above measure is for sentences. Rates of intelligibility for single words is even lower. Which is understandable. A single word without the context of a sentence or phrase is harder to grasp.

Coplan, J., & Gleason, J. R. (1988). Unclear speech: Recognition and significance of unintelligible speech in preschool children. Pediatrics, 82(3 Pt. 2), 447–452.

Hustad, K. C., Mahr, T. J., Natzke, P., & Rathouz, P. J. (2021). Speech Development Between 30 and 119 Months in Typical Children I: Intelligibility Growth Curves for Single-Word and Multiword Productions. Journal of Speech, Language and Hearing Research.

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