What Is Speech Intelligibility?

Speech Intelligibility Defined

a small boy is talking with a man, his speech intelligibility is shown as being imperfectYou can think of speech intelligibility as how well others understand a speaker’s words.  So, when we say a child is 50% intelligible, that means you can understand half of what she is saying.

The development of speech is a gradual process that takes place over time as a child grows older. 

It begins with babbling and progresses to the point where ideally, the child can pronounce all of the speech sounds, resulting in better clarity.

Speech intelligibility also depends on mastering motor control for articulation, volume, prosody, as well as language skills. It is complex! 

In Context/Decontextualized + Familiar/Unfamiliar

To complicate matters, we also think about speech intelligibility in different ways: in context or decontextualized and with familiar or unfamiliar listeners.

It is much easier to understand someone when there is context and you are familiar with them. It makes sense that we understand much more of what a child says during everyday situations, like talking with Dad while playing the 100th round of Spider-Man vs. Gobby!

Speech intelligibility plays a significant role in overall communication effectiveness. To gain a more comprehensive understanding, it is helpful to consider how a child is understood by a variety of different people, not just parents and trained listeners like SLPs.

Should My Child's Speech Be 100% Intelligible?

Most SLPs will reference research done in 1988 (…the year that I was born!) by Coplan and Gleason on parent estimates of conversational speech intelligibility with unfamiliar listeners: 

Age In Years / 4 x 100 = % Understood By Strangers

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

In 2021 Hustad et al. gave us information on decontextualized intelligibility, or how much of what actual strangers understand of a child’s speech in single words and sentences without contextual cues (not just parent estimates):

At 4 years = 50% intelligible

At 5 years = 75% intelligible

At 7+ years = 90% intelligible  

What Causes Poor Speech Intelligibility?

Several factors can impact a child’s ability to speak clearly. These may include neuro-developmental disorders, genetic syndromes like Down syndrome, hearing impairment, cerebral palsy, childhood apraxia of speech, stuttering, delayed or unusual speech error patterns, developmental language disorder, traumatic brain injury, voice disorders, and accents. 

How Do SLPs Measure Speech Intelligibility?

SLPs can measure speech intelligibility in a few different ways. 

The Word Identification Approach to Intelligibility Measurement is a traditional method of assessing speech intelligibility. In this approach, the SLP presents a list of words to the listener and asks them to write down what they hear when listening to the child. The number of correctly identified words is then used to calculate a percentage of intelligibility.

This method is objective and reliable, but it only assesses speech intelligibility on a word-by-word basis, which may not reflect the listener’s ability to understand connected speech in real-life situations and, in my experience, is not often used in school-based practice.

SLPs may also take a recorded speech sample and have an unfamiliar listener write down what they think the child said. The SLP calculates percentage intelligibility based on the number of correctly identified words (or by syllable if being rated by the SLP). This less formal approach can be done with any recorded language sample.

The Scaled Rating Approach is a more holistic method that assesses speech intelligibility. One scale is called the Intelligibility in Context Scale (ICS). This scale has a parent judge how different people understand their child’s speech on a scale of 1 (Never) to 5 (Always). Parents rate themselves, immediate family members, extended family members, child’s friends, acquaintances, teachers, and strangers. A scaled rating approach is more subjective but has shown to be a valid and reliable estimate of a preschooler’s intelligibility (and it’s easy).

Impacts of Poor Speech Intelligibility

Intelligibility norms are just one factor when considering a child’s communication development. It’s also important to assess whether or not your child’s poor speech intelligibility is impacting:

• Their confidence in their voice. If they feel frustrated or embarrassed that people cannot understand them.
• How they engage and participate at school, with friends and family, or out in the community.
• If their speech intelligibility attracts negative social attention from their peers or adults.
• Your child’s teacher is concerned about their speech development.
• If poor speech intelligibility is accompanied by a developmental language disorder, stuttering, reading disorders, or selective mutism.

If any of the above bullet points are true for your child, you should seek an assessment or treatment with an SLP if you haven’t already done so.