The short-term and long-term associations were reported for the main four subscales of the UK- CDI. Overall word knowledge (Comprehension, Production and Phrases Understood) at 12 months
156 correlated with later language but stronger correlations were found for word knowledge at 18 months with later language scores.
Interestingly, UK-CDI word knowledge at 18 months (Comprehension, Production, Phrases Understood) showed highly significant correlations than knowledge at 12 months with later word and language measures at 24 and 36 months. Other research found similar results (e.g., Unhjem et al., 2015).In contrast, Gesture scores were also related to later language but with less strength as children got older. Bavin et al. (2008) found similar results.
Generally, stronger associations were found if the same linguistic category was tested at follow- up. However, there were also few exceptions, for example Comprehension at 18 months was associated strongly with Production at 36 months whilst Production at 18 months was only moderately correlated to Production at 36 months. Other research also suggested that early Comprehension is better than early Production at predicting later Production scores (e.g., Thal et al., 2013). Furthermore, UK-CDI scores correlated better with Toddler CDI questionnaire data rather than in-person testing scores using the PLS-5 UK, this difference was also found by other authors (Guiberson, 2008; P. Lyytinen et al., 1996).
5.3.1. Comprehension
The relationships between Comprehension scores at 12 and 18 as well as between 18 and 24 months were large in strengths and between 12 and 24 months they were moderate. This is supported by other studies which looked at similar age groups (Bornstein et al., 2006; Hamadani et al., 2010; Unhjem et al., 2015).
Our study found weak correlations between Comprehension at 12 months and later Production scores and moderate to weak relationships between Comprehension at 12 months and later grammar scores. This is in line with several other studies that found similar results (P. Lyytinen et al., 1996; P. Lyytinen et al., 1999; Rose et al., 2009; Sachse, Saracino et al., 2007; Unhjem et al.,
157 2015) with few exceptions where Comprehension and later Production correlated moderately in studies with larger samples (Bavin et al., 2008; Feldman et al., 2000).
However, Comprehension tested at 18 months correlated strongly with later Production at 24 and 36 months and strongly to moderately strong with grammar. This result was also found by other studies (Cochet & Byrne, 2016; Friend et al., 2012). In contrast, Production at 18 months correlated only moderately with Production at 36 months. This result is in line with previous research with suggests that Comprehension was somewhat better than Production at predicting later Production scores (e.g., Thal et al., 2013).
Whilst the relationships between Comprehension at 12 months with PLS scores at 18 or 24 months were weak, the association between Comprehension at 18 months and especially Auditory Comprehension at 24 months was strong. This is an interesting finding as it shows that Comprehension (which measures receptive vocabulary) is more stable from 18 months onwards compared to 12 months. Furthermore, the PLS assessment measures more precursor abilities to later language comprehension at 18 months (i.e. play behaviour, understanding of gestures) compared to at 24 months when language comprehension can be measured more directly (i.e. measuring word and language comprehension from questions using syntactically advanced constructions). This suggests that the concepts are more similar for Comprehension at 18 months and PLS Auditory Comprehension at 24 months which could be another reason for stronger associations between these ages.
5.3.2. Production
Production at around 12 months was moderately related to later Comprehension, Production and grammar scores. The results are in line with previous research which looked at early Production and later Comprehension (Korpilahti et al., 2016; Unhjem et al., 2015), later Production (Bavin et al., 2008; Bornstein et al., 2006; Feldman et al., 2000; Hamadani et al., 2010; Korpilahti et al., 2016; Rose et al., 2009; Sachse, Saracino et al., 2007) or later grammar (Feldman et al., 2000; Kreisman,
158 2012). Smaller correlations were expected for Production at 12 months with later language scores compared to other communication variables at 12 months with later language. Our results support this assumption, see Table 22. This is because at 12 months there were still floor effects for Production as children only started to say their first words at that age. According to the literature (Thal et al., 2013), most children with an initial Production delay caught up with their peers at kindergarten age which is how the decreasing correlations with Production at 36 months may be explained. However, these delayed children usually developed grammar more slowly, thus a stronger relationship between Production and later grammar was expected which was confirmed by our study.
Furthermore, Production at 18 months showed mostly strong relationships with the different language variables at 24 months and moderate relationships with language variables at 36 months. Similar results were found in other studies which looked at relationships between Production scores during the second year and later Production (P. Lyytinen et al., 1999; Thal et al., 2013; Unhjem et al., 2015) as well as later grammar scores (Thal et al., 2013). At 18 months, all children in our sample had begun to produce words but the range of ability between children was large. Furthermore, at this age language started to become more stable as all relationships even with in-person assessments showed significant correlations with moderate to large effect sizes.
5.3.3. Gestures
The Gestures and Phrases Understood subscales are usually only used during infancy, whilst vocabulary (Comprehension and Production) can be assessed through to adulthood using standardised assessments (Brownell, 2000a; Brownell, 2000b; Williams, 2007). The strongest correlations were usually found between the same categories according to the literature review (see introduction page 105), hence smaller associations were expected for Gestures and Phrases Understood with language variables at 24 and 36 months compared to Production and Comprehension.
159 The results showed that Gestures correlated moderately with later Gestures scores confirming the results of several other studies (Bavin et al., 2008; Fenson et al., 1994; Fenson et al., 2007). In our study, Gestures were also moderately correlated with Comprehension and Auditory Comprehension scores which were studied up to 24 months. Somewhat lower associations were found between symbolic gestures (selected items from Gestures scale) between 14 and 18 months using an in-person assessment (Laakso et al., 1999; P. Lyytinen et al., 1996). Symbolic gestures are later acquired gestures. This means that at 14 months there might not yet have been a lot of variability between children, explaining weaker associations with later language variables. Furthermore, our data also showed weak correlations between Gestures (at 12 months) and in- person assessments (PLS-5 UK) at 18 months compared to moderate to strong relationships with UK-CDI variables at the same age. Weak correlations were recorded between Gestures and Comprehension if tested between 12 and 36 months (Kreisman, 2012; Rose et al., 2009) and no significant associations were found at older ages (Fish & Pinkerman, 2003). Our correlations between 12 and 18 months were also stronger than between 12 and 24 months. Futhermore, some weak associations between Gestures at 18 months and grammar as well as general language (ASQ) at 36 months were found. This supports the notion that the strengths of associations decrease over time. This is because most of the initially delayed children caught up with their peers at kindergarten age particularly in terms of Production and Comprehension (Thal et al., 2013). Gestures also correlated significantly and moderately with Production between 12 and 18 months and with Production and Sentence Complexity between 18 and 24 months as found by Thal et al. (2013). In contrast to some other studies conducted in Australia, USA and Germany who reported moderate relationships (Bavin et al., 2008; Feldman et al., 2000; Sachse, Saracino et al., 2007), we found a weak correlation between Gestures and Production scores between 12 and 24 months. Furthermore, Gestures were only weakly correlated with language variables at around 3 years. Similar results were found by Rose et al. (2009). Other research has found that Gestures at 15 months were not correlated with risk of language impairment at kindergarten age; however, a Structural Equation Model could show that Gestures had an indirect effect as the influence of
160 Gestures on language impairment risk was mediated by Production at 15 months (Hsu & Iyer, 2016). This indicates that early Gestures should be taken into consideration when aiming to predict language delay in early childhood.
5.3.4. Phrases Understood
When comparing the results of Phrases Understood with the results by Feldman et al. (2000), it was found that the correlations were very similar for Production (.33 vs .31) and somewhat stronger for Sentence Complexity in our study (.35 vs .18) which showed a significant, moderate relationship in comparison to a weak relationship by Feldman (2000). It should be noted that Feldman (2000) looked at a more racially and SES diverse group than used in our sample (see explanation above for Kreisman (2012)). To my knowledge, there is only one more study which looked at Phrases Understood but they tested children at 15 months and at 4 and 5;4 years (Fish & Pinkerman, 2003). Significant and weak relationships were found between Phrases Understood at 15 months and Expressive and Total Language as measured by the PLS-3 at 4 years. This result cannot be directly compared to our findings due to the ages of testing; however, our data indicate moderate relationships between Phrases Understood at 12 and 18 months with language at 36 months thus links with later language seem possible but would need to be tested to be confirmed. Taken together, these results indicate that the early ability of Phrases Understood shows some continuity in terms of later language skills. Therefore, I will include this subscale for establishing the predictive validity of the UK-CDI where possible.
5.3.5. Summary
Three main results stand out: The correlations were usually strongest a) with the same category b) at the closest follow-up testing (all significant and strong); c) correlations weaken with increasing age gap between testing times. Other studies (Bavin et al., 2008; Feldman et al., 2000; P. Lyytinen et al., 1996) came to the same conclusion. In addition, the strengths of associations within the same
161 category generally decreased over time. This also supports the results by Unhjem et al. (2015) who found a decrease in the strength of association for the same category over time for the control group representing typically developing children in contrast to the at-risk group who maintained high associations over two testing intervals (each 3 months apart).
In addition, UK-CDI scores taken at around 12 months showed stronger associations up to 24 months compared to 36 months. However, it should be noted that Phrases Understood and Production at 12 and at 18 months correlated significantly and moderately with grammar scores at 36 months - in other studies this was only assessed for Production up to 36 months, and similar results were obtained (Thal et al., 2013).
The data also show that language was more stable from 18 months onwards as the correlations were consistently significant with scores at 24 and 36 months. Other research has also shown that correlations were stronger when children were older at the first time of testing (Cochet & Byrne, 2016; Friend et al., 2012).
Taken together, results of the newly established UK-CDI largely support the findings of other CDI predictive validity studies. This is the first time the UK-CDI has been used to measure correlations with later language skills, and it is important as the UK-CDI is the only comprehensive communicative inventory which has been normed and standardised for the British population. In addition, this study adds value to the research field as it includes all relevant CDI: Words and Gestures scales (including Phrases Understood) and regular testing points using a variety of parent report and in-person assessments whilst other studies often used fewer testing points and a smaller variety of testing materials.
The data suggests a strong continuity between time points particularly between the UK-CDI scores at 18 months and later language scores. For this reason, I will also examine if these UK-CDI scores can be used to make predictions about later language delay for individual children.
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