Not all hits in our literature search fulfil the inclusion criteria in our systematic vocabulary intervention review

In our literature review we want to find out about vocabulary intervention approaches, so we know what approaches have been used in earlier research and what works best with regards to improving vocabulary in children with Down syndrome.

The second article that we have selected in this series of our literature review posts is ‘Advancing imitation and requesting skills in toddlers with Down syndrome’ by Feeley, Jones, Blackburn, & Bauer (2011). This study was a hit in our literature search, however, the study will not be included in our review because it did not meet our inclusion criteria; this is since it is, a.) Not directly a vocabulary intervention, and b.) Not including a control group/condition. As the study was a hit in our search we will nonetheless discuss the study in this post, in relation to variability in intervention content and research quality across the various hits in our search.

The reason we are including only studies with control groups/conditions is a really important one; if children receive an intervention and their performance improves, but there is no control group or control condition, then it is not possible to determine whether they improved simply due to attention, time in general, or any given property of the specific intervention. Also, without comparing to another population, it is not possible to argue that the intervention is specifically effective for the population tested, e.g., Down syndrome. Without a control condition or control group researchers are unable to empirically test the theory underlying the given intervention.

The article by Feeley et al. (2011) aimed to investigate the result of an intervention program developed for children with Down syndrome that focuses on imitation and requesting. Four toddlers who have Down syndrome participated in their study. Due to the suggestion of relative strengths in social development in Down syndrome, based on a review by Fidler et al (2005), social interaction was used in the intervention by Feeley et al. to encourage and reinforce requesting and verbal imitation in toddlers with Down syndrome. The intervention involved providing specific prompts (telling the child to say a certain sound) to give the child opportunities to verbally imitate or encourage them to request. Corrective feedback was also given, along with social reinforcement such as hugging and social praise.

The sessions were run by the child’s speech therapist and their mother. Each intervention session lasted around 15 minutes, with 3 sessions per week. Sessions could involve up to 10 ‘opportunities’ (meaning opportunities to respond to prompts), but often consisted of between 5 and 7 opportunities. It was unclear why this variation was present rather than a set number of opportunities in each session for each child. Verbal imitation and requesting interventions were done at the same time so that the children could use any learnt verbal imitation skills when trying to request. For verbal imitation children were presented with a model of the target sound (spoken by the interventionist – mother or speech therapist), such as the ‘ahhhh’ sound. In earlier sessions the sound was exaggerated, the child was also given a physical prompt (by having gentle pressure on their chin). Over the course of the sessions the interventionist made less exaggeration of the target sound and put less pressure on the child’s chin. For the requesting training, the training followed a sequence, starting with training the use of a gaze shift to request social interaction with an object (the interventionist would prompt the gaze shift by, for instance, taking a deep breath or moving into the child’s line of sight). If the child looked at the object and then to the interventionist they were subsequently given the object. Training next moved on to the use of gaze shift and any vocalisation (whereby the interventionist additionally prompted the child by gently tapping on their mouth). Finally gaze shift was to be accompanied with an approximation of the appropriate requesting word (e.g., an ‘mmm’ sound for ‘more’). The interventionist’s prompt was to tell the child to ‘say ‘mmm’ for instance, and the object was placed in a jar such that the child was requesting for help (e.g., to get the object from out of the jar).

The children were assessed in terms of whether they reached a mastery criterion, e.g., 80% independent correct responding (independent correct responding refers to children’s ability to imitate the sound made by the interventionist without the need to hear an exaggerated verbal model and/or a physical prompt, and their ability to make a request without a prompt) in ‘two consecutive sessions on two days’ (pg. 2418), we assume this means a correct response one day and again the next day. The authors found that the children were able to acquire verbal imitation and requesting skills. Each child’s results are reported separately in terms of how many probe opportunities it took to reach the mastery criterion. It is difficult to see how many probe opportunities were needed on average as group level results are not provided.

Maintenance of skills was assessed via a follow up (3 to 4 weeks after intervention). The follow up was based on giving the child just one opportunity to respond to a probe, e.g., prompting the child to ‘say ‘mmm’, and seeing whether the child responds correctly in that one instance, or e.g., showing the child the object in the jar such that they are prompted to make a request for help getting the object out of the jar. The children were probed once for each of the sounds that they had learnt in the intervention and the most sophisticated form of requesting they had learnt in the intervention. For three of the four children follow up was assessed and showed that children continued to show the skills (i.e., same level of performance) that they showed at the end of the intervention.

Generalisation was not measured in a systematic way, e.g., a particular set of non-trained items tested across each child.  Rather, sounds that the interventionists said that they heard spontaneously during intervention, but that the child could not say at baseline, were tested again after the intervention. Generalization was assessed for just 2 of the 4 children and showed that they generalized to novel sounds. Given that this was only 2 children it does not tell us about what we should expect in the Down syndrome population more generally.

The authors note that they have demonstrated the effectiveness of the intervention, however we think this is a rather strong statement given the design and the limitations of the study. Improvement could be due to natural development over time rather than an effect of the intervention. The authors note the need for replication, and a replication was in fact subsequently carried out by Bauer and Jones (2014); we will discuss this study in another post in this series. However, we would argue that it is much more than replication that is needed here. The findings do not give us any idea of meaningful group level patterns for those with Down syndrome. A follow up intervention study would be needed to test the claims made by Feeley et al, but this should involve a larger sample and a control comparison in order to tell us something more meaningful about this intervention, whether it results in any significant effects, and test the validity of the theory underlying it.

Further, for the four children in the study it was difficult to compare individual differences in abilities. In the initial descriptions of the four children different measures were reported for the respective children. We were unclear about why the different measures for different children were reported. Other issues, such as one child missing a part of the intervention due to interventionist’s time constraints, and one child’s parents introducing a different intervention for one of the sounds, present a number of confounds in terms of interpreting the results in this very small scale study.


Fidler, D. J. (2005). The emerging Down syndrome behavioral phenotype in early childhood: Implications for practice. Infants & Young Children, 18(2), 86-103.

Feeley, K. M., Jones, E. A., Blackburn, C., & Bauer, S. (2011). Advancing imitation and requesting skills in toddlers with Down syndrome. Research in developmental disabilities, 32(6), 2415-2430.


Liz and Kari-Anne

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