Intelligibility

Also called Speech clarity, Speech intelligibility, Being understood

How much of a person's speech a stranger understands. It has an expected trajectory in children, and a practical answer when it stays low.

What it is

Intelligibility is the proportion of someone's speech a listener can understand. It's usually judged by a stranger, because parents understand far more than anyone else and their estimate runs high. It's a whole-person measure: sounds, prosody, pace, volume and grammar all feed into it, and a listener who knows the topic understands more than one who doesn't.

It's not the same as having correct speech sounds. A child with several immature sounds can be perfectly intelligible, and an adult with mild dysarthria can be hard to follow with every sound technically present.

Who it can help

Parents wondering whether a child's speech is on track, and anyone supporting someone whose speech is unclear for any reason: speech sound disorder, cerebral palsy, Down's syndrome, Parkinson's, stroke.

The honest thing to say is that low intelligibility is exhausting for the speaker, who has to repeat, and that a backup is not giving up on speech. Many people who speak use a board or device for the moments that matter, and are better understood and less tired for it.

How it works

The commonly used guide is that a child should be mostly understood by a stranger by around three, and almost fully understood by around four, even though some individual sounds (r, th, and clusters like "str") are still developing at five and beyond (speech sound ages). Familiar adults understand more, earlier. If a stranger understands little of a child's speech at three, or a child is frustrated at not being understood at any age, that's a reason to ask for an assessment.

Therapists measure it by rating a sample, sometimes by asking a listener to write down what they heard and scoring the match. For adults with progressive conditions, intelligibility is tracked over time to decide when to introduce or step up AAC, and to plan voice banking while speech is still clear.

Listeners matter as much as speakers. Knowing the topic, seeing the face, a quiet room and a patient partner all raise intelligibility without the speaker changing anything.

Getting started

Ask a stranger, or someone who sees the child rarely. What fraction did they follow? That's the number that matters, not yours.

Change the listening conditions before the speaker: turn off the telly, face them, know the topic ("we're talking about school"). Ask for the first letter, or the category, rather than "say it again" for the fourth time (repair strategies).

Keep a low-tech backup for the moments that fail: a communication book, or a phone with the Talk board loaded, which speaks a typed or symbol message aloud in a voice the person chooses. Being understood at the pharmacy is worth more than a principle.

Where to get help

Speech and Language UK explains what clarity to expect at each age. Ask for a speech and language therapy referral if a three-year-old is hard for strangers to follow. For adults with unclear speech, the Stroke Association, Parkinson's UK and the MND Association each have plain guides.