Table 1 - Intelligibility

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Bowen, C. (2011). Table1: Intelligibility. Retrieved from on [insert the date that you retrieved the file here].

Table 1: How well words can be understood by parents
By 18 months a child's speech is normally 25% intelligible
By 24 months a child's speech is normally 50 -75% intelligible
By 36 months a child's speech is normally 75-100% intelligible

Lynch, Brookshire & Fox (1980), p. 102, cited in Bowen (1998).

The proportion of a speaker's output that a listener can readily understand

The term intelligibility refers to 'speech clarity' or the proportion of a speaker's output that a listener can readily understand. In typical development, as children learn to talk, their comprehensibility to those around them steadily increases. A key characteristic of children with speech sound disorders is that they are often significantly less intelligible than non-speech-impaired children of the same age.

In young children there is often quite a marked difference between single word (SW) and conversational speech (CS) intelligibility; between intelligibility to their close family members and intelligibility to unfamiliar listeners; and intelligibility in known versus unknown conversational topics. With regard to families, siblings may sometimes be more adept than parents in comprehending what their little brothers and sisters are saying.

Weiss (1982): 24-36 months

An early source of typical intelligibility criteria came from Weiss (1982) who suggested that speech should be:

26-50% intelligible by 2;0
51-70% intelligible by 2;6
71-80% intelligible by 3;0

Intelligibility to parents: 18-36 months

Table 1, above, provides a rough guide to how clearly a child should be speaking in the age-range 18 to 36 months. It is important to bear in mind that there is considerable individual variation between children. If, as a parent, you are in doubt about your own child's speech sound development or speech clarity, an assessment by a speech-language pathologist / speech and language therapist (SLP/SLT) will quickly tell you if your child is 'on track' and making the right combination of correct sounds and 'errors' for their age.

Intelligibility to strangers 12-48 months



Flipsen Jr. (2006) wrote, "Currently the only reliable data on the conversational intelligibility of children that appear to exist reflect parent report estimates (Coplan & Gleason, 1988), and these have yet to be validated against more objective measures." Nonetheless, some SLPs/SLTs use the Coplan and Gleason "formula" as a guide to the expected conversational intelligibility levels of preschoolers talking to unfamiliar listeners, or "strangers". This is it:

Child aged 1;0 = 1/4 or 25% intelligible to strangers
Child aged 2;0 = 2/4 or 50% intelligible to strangers
Child aged 3;0 = 3/4 or 75% intelligible to strangers
Child aged 4;0 = 4/4 or 100% intelligible to strangers

Pascoe (2005) is in general agreement, and says, "By three years of age, a child's spontaneous speech should be at least 50% intelligible to unfamiliar adults"... "By four years of age, a child's spontaneous speech should be intelligible to unfamiliar adults, even though some articulation and phonological differences are likely to be present."




Below 66% percent

Gordon-Brannan & Hodson (2000) determined that children above the age of 4;0 with speech intelligibility score below 66% should be considered as candidates for intervention. What this means is that if fewer than 2/3 of the utterances of a child aged 4;0 in conversation with an unfamiliar listener can be understood by that listener, then intervention is indicated. Unfamiliar listeners should be able to understand at least 66% of what a child of 4;0 says.

Intelligibility Rating Scale

In the Quick Screener child speech assessment procedure is a simple, subjective, impressionistic (so unreliable!) 5-point conversational speech intelligibility rating scale is used. It is useful to have an intelligibility rating from a child's parent or parents, SLP and a 'stranger' (unfamiliar listener). The scale is:

1: completely intelligible in conversation
2: mostly intelligible in conversation
3: somewhat intelligible in conversation
4: mostly unintelligible in conversation
5: completely unintelligible in conversation

While these ratings are unreliable they are useful clinically as a means of comparing impressions of intelligibility in the same child over time and between 'raters'.


Bowen, C. (1998). Developmental phonological disorders: A practical guide for families and teachers. Melbourne: The Australian Council for Educational Research Ltd.

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

Flipsen, P., Jr. (2006). Measuring the intelligibility of conversational speech in children. Clinical Linguistics & Phonetics, 20(4), 303-312.

Gordon-Brannan, M., & Hodson, B. (2000). Intelligibility/Severity Measurements of Prekindergarten Children's Speech. American Journal of Speech-Language Pathology, 9, 141-150.

Lynch, J.I., Brookshire, B.L., & Fox, D.R. (1980). A Parent - Child Cleft Palate Curriculum: Developing Speech and Language. Tigard, OR: CC Publications.

Pascoe, M. (2005 May). What is intelligibility? How do SLP's evaluate and address children's intelligibility intervention? The Apraxia-Kids Monthly, 6, 5.  Accessed June 2018.

Schiavetti, N. (1992). Scaling procedures for the measurement of speech intelligibility. In Kent, R. D. (Ed.), Intelligibility in speech disorders.(pp. 11-34). Philadelphia, PA: John Benjamins.

Weiss, CE (1982). Weiss Intelligibility Test. Tigard, OR: CC. Publications.

See also

Table 2 Phonological Processes

Table 3 Elimination of Phonological Processes

Table 4 Phonetic Development

Inconsistency in Child Phonology

Speech and Language Development Index