The thing that most often brings parents to us for this particular profile isn't a single persistent error — it's the variability. "She said it perfectly yesterday, and now it's gone again." "He can say his own name clearly on Monday and three different ways on Tuesday." It's the unpredictability that wears families down, because it makes it almost impossible to know what to practise, what to expect, or whether progress is happening at all.
This article is about that particular picture — what speech pathologists call inconsistent speech sound production. It's one profile within the broader group of speech sound disorders, and it's distinct from a delay (where a child is on the typical developmental path, just behind schedule — see Delayed Speech Sounds at Six) and distinct from consistent atypical errors (where the errors are unusual but at least predictable — see Atypical Speech Sound Errors). What makes this profile different is that the errors themselves change, and that tells us something important about where the difficulty lies.
Let's break down what this means, how it's identified, and why it matters.
Where "Inconsistent" Sits Among Speech Sound Disorders
We want to be upfront about the language here. "Speech sound disorder" is the umbrella term for any speech difficulty that's more than a mild delay, and it covers several quite different profiles, including the consistent atypical errors in our other article. The word can feel heavy, and we understand that. "Inconsistent" is the label for one profile under that umbrella, and it tells us something important about how the child's speech system is working, which guides us toward the right kind of help.
It doesn't mean there's something fundamentally "wrong" with your child. It means their brain is organising speech production in a way that makes it harder for them to produce sounds and words the same way each time, and they need a specific kind of support to build those skills.
How Is Inconsistent Speech Different from Delayed or Atypical Speech?
Professor Barbara Dodd's classification model for speech sound difficulties provides a helpful framework for understanding the differences. Dodd's model, which is widely used in Australia and internationally, identifies four main categories (Dodd, 2005):
| Category | What It Means | Key Feature |
|---|---|---|
| Articulation difficulty | A child can't produce a specific sound correctly due to motor or structural factors | The same sound is always affected in the same way |
| Phonological delay | A child uses typical developmental patterns but hasn't grown out of them at the expected age | Patterns are familiar and expected — just lingering |
| Consistent atypical errors | A child uses non-developmental patterns — errors that aren't part of the typical pathway | Errors are unusual but consistent — the same error happens every time |
| Inconsistent speech sound production | A child produces the same word differently on different occasions | Inconsistency is the defining feature |
Inconsistency as a Hallmark
Inconsistency is the defining feature of this profile, and it's what sets it apart from delayed and atypical patterns.
All children are slightly inconsistent with their speech sometimes — especially when they're learning new words or when they're tired. But children with an inconsistent profile show a level of inconsistency that goes well beyond what's typical. In assessment, speech pathologists measure this with a set task rather than by impression. On the DEAP, for example, the child names the same 25 pictures three times over, with other activities in between, and we compare the three attempts at each word. If 40 per cent or more of the words come out differently across the three attempts, the child's speech is classed as inconsistent (Dodd, 2005). That result is read alongside everything else in the assessment, not on its own, and it's what points us toward a different therapy approach from the one we'd use for delayed or atypical profiles.
Some of the children I've worked with who sit in this inconsistent group are the ones whose families describe the "good day, bad day" pattern most vividly — and naming the inconsistency often comes as a relief, because it finally explains what they've been seeing. Why does inconsistency matter? Because it points to where in the chain of producing a word the difficulty sits. Dodd's research suggests that for most children in this group the problem is phonological planning: choosing and assembling the right sounds for a word before the mouth ever moves, so the child has no stable plan to draw on and works the word out afresh each time. A smaller group have a motor-planning difficulty, childhood apraxia of speech, where the plan for the movements themselves is unreliable. The two can look identical to a listener, and they need different therapy, so telling them apart is one of the main jobs of the assessment.
Childhood Apraxia of Speech: One Example
The best-known cause of inconsistent speech, though not the most common, is childhood apraxia of speech (CAS). CAS is a motor speech difficulty where the brain has trouble planning and coordinating the precise, rapid movements of the tongue, lips, jaw, and palate needed for clear speech.
Children with CAS often show:
- Inconsistent errors — the same word comes out differently each time
- Difficulty with longer or more complex words — errors increase as words get longer
- Groping or searching movements — the child may visibly try to position their mouth before speaking
- Unusual prosody — their speech may sound flat, choppy, or have unusual stress patterns
- Limited consonant and vowel repertoire — they may use fewer different sounds than expected
- Difficulty imitating sounds or words on request
CAS is relatively uncommon, affecting an estimated 0.1 to 0.2 per cent of children, though exact prevalence figures are difficult to establish (Morgan & Webster, 2018). It's important to note that CAS is not a "one size fits all" diagnosis — children with CAS vary widely in severity and in how their speech presents.
In Australia, researchers at the University of Sydney have contributed significantly to our understanding of CAS. The work of Professor Patricia McCabe and her colleagues has been particularly influential — her team's systematic reviews and treatment research have shaped how Australian speech pathologists assess and treat children with CAS (Murray, McCabe & Ballard, 2014). Morgan and Webster's (2018) research has also helped establish diagnostic criteria for paediatricians and clinicians working with these children.
The American Speech-Language-Hearing Association (ASHA) has published a position statement on CAS, and Speech Pathology Australia (SPA) has a fact sheet for families; both emphasise that:
- Diagnosis should be made by a speech pathologist with expertise in motor speech difficulties
- CAS requires a specific, evidence-based intervention approach
- Therapy should be intensive and frequent
- CAS is a motor speech difficulty, not a language or cognitive difficulty (though these can co-occur)
It's also worth emphasising that CAS is not the only cause of inconsistent speech production. Some children show significant inconsistency without meeting the full criteria for CAS. Dodd's model captures this distinction — a child can have an inconsistent phonological disorder without having CAS, and in Dodd's caseload studies that was the more common picture. Telling the two apart matters because inconsistent phonological disorder responds to the core vocabulary approach, while CAS needs motor-based therapy; we explain both in the companion article.
What Does Inconsistent Speech Look Like Day to Day?
For parents, inconsistent speech production can be particularly confusing because it's unpredictable. On a good day, your child might say a word clearly, and you think they've "got it" — only for them to produce it completely differently the next day. This inconsistency can make it hard to track progress and can be frustrating for both you and your child.
You might also notice:
- Your child is much harder to understand than other children their age
- They struggle more with longer or unfamiliar words
- They may become frustrated or give up when trying to say something difficult
- Their speech may seem to get worse when they're tired, excited, or under pressure
- Other people — teachers, grandparents, peers — have difficulty following what they're saying
If this sounds like your child, a comprehensive speech pathology assessment is an important first step.
A quick thought on hearing. Before diagnosing a speech sound disorder, it's important to be sure a child is hearing clearly. A lot of the sounds that get muddled — "s", "sh", "f" — sit right in the range that's most affected by things like glue ear. An audiologist visit is a straightforward first step.
Why Early, Accurate Identification Matters
Getting the right assessment and the right diagnosis makes a real difference for children with inconsistent speech. The contrast-based therapy that works for phonological delays and consistent atypical errors is not the right fit for inconsistent profiles (Crosbie, Holm & Dodd, 2005). Children with inconsistent phonological disorder do best with the core vocabulary approach, which builds a stable way of saying whole words. Children with CAS need motor-based therapy that trains the movements. The earlier the right work begins, the better.
An accurate assessment also helps set realistic expectations. Inconsistent speech production often requires more frequent and intensive therapy than a straightforward delay, and progress can be slower — but with the right approach and consistent support, children do make meaningful gains.
If your child is very hard to understand and you've noticed that their speech errors seem to change from day to day, getting an accurate assessment is the first step. The right diagnosis leads to the right therapy — and the right therapy leads to real progress. Speaking Speech Pathology offers mobile speech pathology in your home across Brisbane's south side and Logan. get in touch with our team to find out more. Any actual clinical work — assessment, diagnosis, or therapy — happens in a consultation about your child, not from an article.
Ready for practical strategies? Read our companion article: Therapy for Childhood Apraxia of Speech and Disordered Speech: What Works in Brisbane
Alexandra Bouwmeester is a Senior Speech Pathologist (MSPA, CPSP) with over 14 years' experience assessing complex speech sound presentations. She uses Dodd's classification framework and stays current with Australian research on CAS and disordered speech production.
References
- Crosbie, S., Holm, A., & Dodd, B. (2005). Intervention for children with severe speech disorder: A comparison of two approaches. International Journal of Language & Communication Disorders, 40(4), 467–491.
- Dodd, B. (Ed.). (2005). Differential diagnosis and treatment of children with speech disorder (2nd ed.). Whurr.
- Morgan, A. T., & Webster, R. (2018). Aetiology of childhood apraxia of speech: A clinical practice update for paediatricians. Journal of Paediatrics and Child Health, 54(10), 1090–1095.
- Murray, E., McCabe, P., & Ballard, K. J. (2014). A systematic review of treatment outcomes for children with childhood apraxia of speech. American Journal of Speech-Language Pathology, 23(3), 486–504.
