Complexity Approach: Clusters

Information on the 2/3-element clusters procedure and available materials

The use of 2 or 3 element consonant clusters (i.e., /fɹ/, /pɹ/, /spɹ/ etc.) in word-initial position in 16 real words or non-words.

Suitable children

  • Consistent phonological impairment
  • Severity: moderate - severe
  • Age: between 2;08 - 7;11 years, with most children being around 4 years
  • The exclusion of at least 5 - 7 speech sounds from their phonetic/phonemic inventories

(Williams et al. 2010, Gierut 1989, 2008)

Clusters procedure

(Adapted from Gierut and Champion 201)

  1. After following the target selection procedures to identify one 2/3-element cluster, follow Storkel's (2018b) evidence-based advice on the optimal combination of vocabulary characteristics to best promote generalisation of learning: high frequency + high density; low frequency + high density; high frequency + mixed density; low frequency + later acquired; or nonwords. Note here, that density refers to the number of other words in the language that sound like each other  i.e.,  words from high density neighbourhoods contain lots of words that sound similar to one another (bat, mat, fat, rat, sat, cat.......) while words from low density neighbourhoods do not have many other words that sound like them (and are also often less frequent) (umbrella).
  2. Sixteen words or nonwords should be developed to make the training set.
  3. Note: these words/nonwords are not made up into minimal pairs in this approach i.e., only target words are focused on. This means that the 16 words/nonwords would all be target words without a minimal pair contrasting the target with the child's realisation.
  4. If nonwords are to be used, the child is familiarised with them and they are assigned meaning through storytelling (i.e., via aliens, mermaids, objects or actions etc.).
  5. Imitation phase: the child imitates the SLTs' model for each word/non-word. The child is praised for correct imitations and provided with graded modelling for incorrect attempts (see table 1 below). Once the child receives 75% over 2 consecutive sessions, or when 7 sessions have been completed, they can move on to the next therapy stage.
  6. Spontaneous phase: the child produces the target words/non-words without a model and is praised throughout attempts. Once 90% accuracy has been achieved over 3 sessions, or when 12 sessions have been completed, the child can progress to the next stage of therapy.
  7. Generalisation probe: A generalisation probe (i.e., using non-treated words/simple phrases which include the target in the treated position) is administered. For this approach, this will look at implicational changes (i.e., the emergence of system-wide maturation in the child’s speech sound system below the complexity level of the clusters targeted (see NOTE 2 below)).
  8. At this point there are two options: (1) another therapy target may be chosen or; (2) the child may be discharged. SLTs should use their own clinical judgement here.

Note: For this intervention, a 'trial' is considered to be the production of all 16 words/non-words.

Note:  Generalisation of the target cluster to spontaneous speech is not expected with this approach. Instead implicational improvements to less complex aspects of the child’s speech sound system are expected. Find out more about on our target selection page.

Table 1: Graded Modelling during Imitation stage (Adapted from Gierut and Champion, 2001)
1st trial of a sessionEmphasise sequencing the full 3-element cluster[skwxxx]
2-3 trials of a sessionEmphasise sequencing the first two sounds of the 3-element cluster[skwxxx]
4-5 trials of a sessionEmphasise sequencing last two sounds of the 3-element cluster[skwxxx]
Final trial of a sessionEmphasise sequencing the full 3-element cluster[skwxxx]

Note: Clinical judgement should be used alongside the child's assessment data when choosing and implementing an intervention protocol.


  • Probes can be used to monitor the child's progress with an intervention and detect generalisation.
  • Around 40 probe words should be used for each target.
  • Probes should include words that contain the target sound in various word positions and across sound structures.
  • Probes should also consider generalisation from single words to conversational speech.

(Williams et al. 2010)

Supporting literature


Intervention intensity

Find out more information on our intervention intensity page on the empty set approach.