Further analysis was conducted to investigate the current UK-CDI data in terms of ability groups and language stability over time. Research has shown that the prevalence of children between 2 and 3 years with significant language delays not due to underlying hearing impairments, neurological or emotional disorders or genetic syndromes is between 15% - 20% (Horwitz et al., 2003; Reilly et al., 2007). At 5 years about 6.8% of children still have a language deficit (for a review see Law, Boyle, Harris, Harkness, & Nye, 2000b, see also Introduction, pages 5 - 6). This decrease of language deficits over time has to be taken into account when aiming to detect children with later language delay/disorder during early development. During the early stages of language development (around 12 months), there is little variance in language production scores. However, shortly after that period and throughout toddlerhood language ability is characterized by a high variability between children.
Due to this smaller variance in early language, children were grouped into quartiles (at 12 and 18 months) to distinguish them by ability. This choice of cut-off was adopted following criteria by a recent study by Duff et al. (2015) who also classified British English children aged 18-19 months as late-talkers if they fell below the 25th percentile on the Oxford CDI. Thus, for this next analysis, I will
use quartile groups which may show a clearer developmental picture and stronger stability over time than looking at small groups at the extreme ends of the spectrum only who show some stability, with suggestions that high language ability (at or above 90th percentile) at early stages is
more stable over time (Thal et al., 1997). It is important to follow up on Duff’s approach to investigate if language is also stable if children are grouped into wider groups (e.g. at or below the 25th percentile rather than at or below the 10th percentile) and for groups with average ability to
include a larger amount of children overall.
Given previous research and the results presented in Chapter 5, Analysis 1, above, the hypotheses were as follows:
163 1) Children from the higher language ability groups continue to have higher language scores at later timepoints and that children from medium and lower ability groups continue to have lower language scores than high ability children.
2) High language ability level should be more stable over time than the other ability groups. This means children with high abilities during early communication development are more likely to remain advanced in their language skills when tested at older ages. In contrast, children with typical or slow initial language show less continuity over time and thus are more likely to change group membership.
3) More stability was expected within the same language categories compared to between categories (e.g. Production ability was expected to have a significant effect on later Production scores).
4) Better prediction of scores was expected using Comprehension and Production than Gesture ability groups.
5) More stability was expected from ability groups at 18 months as opposed to ability groups at 12 months.
6.1. METHOD
6.1.1. Participants
Participants with missing values (7%) were omitted so that the data set included 67 children41.
The children (28 girls) were aged 11.58 months on average at Time 1 (SD = 1.06, range = 10 - 15 months). Levels of ability were created from the UK-CDI preliminary norms (Alcock et al., 2017) for Gestures, Production and Comprehension at 12 and 18 months. For example, the preliminary norms
41 In total 15 children were not taken into account for this analysis. Ten children were 19 months at Time 2
but UK-CDI (preliminary) norms are for children between 8-18 months. Due to lack of reference data, these children could not be put into ability groups. In addition, 2 children did not take part in the PLS test at 18 months and 3 other children did not take part in PLS assessment at 24 months. Out of the 10 children who were too old for this analysis, 2 parents did not complete the gesture subscale of the UK-CDI at 18 months and 1 child did not take part in the PLS assessment at 18 months and another child did not take part in the assessment at 24 months.
164 show the different cut-offs points per age group and category (at 12 months for Gestures: 25th
percentile: 16 gestures, 50th percentile: 22 gestures and 75th percentile: 28 gestures). The children
from the current data set were put into one of the quartile groups according to the prelimary norm cut-off points depending on the amount of gestures used and the age of the child, see Table 23 below.
We would expect around 16 – 17 children per quartile group if the data was equally distributed across groups and similar to the preliminary norm data. From Table 23, we can say that the data was similar to the norm data as our data was not consistently over-represented in one group. This similarity between our sample and the norm sample may help to generalize our findings to the UK population.
Table 23. Distribution of children into 4 ability groups per UK-CDI measure by percentile ranks (according to preliminary norms)
Children (N = 67) distributed into 4 ability groups per measure (using percentile ranks)
Measures 1st-25th (low ability) 26th-49th (low- average ability) 50th-74th (average- high ability) 75th-99th (high ability) Gestures 12m 11 15 17 24 Comprehension 12m 17 23 17 10 Production 12m 17 15 20 15 Gestures 18m 12 18 19 18 Comprehension 18m 22 16 15 14 Production 18m 15 21 16 15
There is a certain overlap and movement between the groups. For example at 12 months, out of the 17 children with slow Production development (1st-25th percentile group), seven children also
had small Comprehension skills (of which 3 had slow development in terms of Comprehension and Gesture) and one child also had slow Gesture development but typical Comprehension skills. At 18 months, out of the 15 children with a productive delay at 18 months, 10 also had a comprehension delay (of which seven had a combined gesture and comprehension delay). Furthermore, nine of the
165 children with a Production delay were already in the lowest ability group for Production at 12 months. This means about half the children (52.9%) remained slow developers. This proportion was similar for children with sustained Comprehension delay (58.8%) and somewhat less strong for Gesture delay (36.4%). This shows that many children with a delay at 18 months already had low scores six months earlier. Next, I will examine the stability of group ability separately for the different UK-CDI categories (see Table 23). It is also important to note that slow developers often have problems in more than one domain (e.g., Thal et al., 2013).
6.1.2. Materials
The same language tests were used as described above.
6.1.3. Procedure
The procedure was the same as described above.
6.1.4. Statistical analysis
In the following analyses, the z-score transformed language scores were used as above and where necessary the two-step transformed scores were employed- also transformed into z-scores. Missing cases were removed from the analysis.
A first analysis assessed if the groups differed in terms of gender or maternal education (see data in Appendix 12). Chi-square tests of independence were used separately for gender and maternal education with the separate UK-CDI scores (Comprehension, Production and Gestures) at 12 and 18 months. There were no statistically significant differences between the ability groups in terms of gender. Level of maternal education (i.e. low (no qualification to A Level or similar) or high (University degree or similar)) was used as categorised by Fenson et al. (2007). There were no
166 statistically significant differences between the ability groups in terms of maternal education in our data (see Appendix 12 for results).
Multivariate analyses of variance (MANOVAs) were employed to assess if communication ability level had a significant effect on earlier, concurrent or later language scores42. MANOVAs were
chosen as the data met the assumptions for this type of analysis. The analyses include a range of dependent variables (N = 15, see Table 24 below) and due to the limited sample size it was decided not to compromise the analysis by including covariates.
Table 24. Dependent variables for different MANOVAs
Dependent variables 1. Comprehension 12m 2. Production 12m 3. Gestures 12m 4. Comprehension 18m 5. Production 18m 6. Gestures 18m 7. PLS Auditory Comprehension 18m 8. PLS Expressive Communication 18m 9. Comprehension 24m 10. Production 24m 11. Sentence Complexity 24m 12. PLS Auditory Comprehension 24m 13. PLS Expressive Communication 24m 14. ASQ Communication 36m 15. Production 36m 16. Sentence Complexity 36m
In the following, one of the six independent variables of ability group (i.e. ability group of Gestures at 12 and 18 months, ability group of Comprehension at 12 and 18 months and ability group of Production at 12 and 18 months) was used per MANOVA. The dependent variables were the same for all MANOVAs (see Table 24 above), except for the one variable from which the individual ability subgroup (see Table 23 above) was derived (e.g. the continuous variable of
42Future analysis could use multilevel/mixed effects models for this repeat-measures design. This would
be worthwile as children could be included in the analyses even if they did not take part in all testing stages as these models can account for dropout bias.
167 Gestures at 12 months was removed when the independent variable of Gesture ability group at 12 months was used).
The MANOVA was then followed-up with Tukey HSD multiple comparisons to test which groups (1st-25th percentile, 26th-49th percentile, 50th-74th percentile, 75th-99th percentile) were significantly
different from each other. Some variables were not normal (Expressive Communication at 18 months, Sentence Complexity at 24 and 36 months and ASQ at 36 months) even after 2-step transformation but they were still included in the MANOVA.
6.2. RESULTS
6.2.1. Production
A MANOVA was conducted with Production ability at 12 months (IV) and the different communication variables (DV), see Table 24 except for Production at 12 months. The ability to produce words at 12 months as defined by Production scores (see Table 23 above) had a significant effect on concurrent and later language scores (Pillai’s trace V = 1.0, F (45,153) = 1.71, p = .009), see Appendix 13. However, separate univariate ANOVAs of ability group by single language measures showed that ability to produce words at 12 months had a significant effect on many dependent variables which demonstrated large effect sizes, as shown in overview in Table 25 below.
Production ability at 12 months had a significant effect on Comprehension scores but not on Gesture knowledge at the same age. Furthermore, Production ability group at 12 months also had a significant effect on Comprehension at 18 and 24 months and Production scores at 18, 24 and 36 months. In addition, these early Production ability groups had an effect on expressive language six months later and even on grammar scores (Sentence Complexity) up to two years later.
168 Table 25. Separate univariate ANOVAs between ability groups at 12 months (defined from Production at 12 months)
and the communication variables (see Appendix 13)
Independent variable F Sig.
Partial Eta Squared Ability groups for
Production at 12 months Comprehension 12m F(3,63) = 8.29 .000 .283 Gestures 12m F(3,63) = 1.40 .250 .063 Comprehension 18m F(3,63) = 8.04 .000 .277 Production 18m F(3,63) = 7.62 .000 .266 Gestures 18m F(3,63) = 0.93 .432 .042 PLS Auditory Comprehension 18m F(3,63) = 1.12 .347 .051 PLS Expressive Communication 18m F(3,63) = 3.65 .017 .148 Comprehension 24m F(3,63) = 5.95 .001 .221 Production 24m F(3,63) = 3.34 .025 .137 Sentence Complexity 24m F(3,63) = 5.97 .001 .221 PLS Auditory Comprehension 24m F(3,63) = 0.53 .661 .025 PLS Expressive Communication 24m F(3,63) = 2.43 .074 .104 ASQ communication 36m F(3,63) = 0.96 .418 .044 Production 36m F(3,63) = 3.40 .023 .139 Sentence Complexity 36m F(3,63) = 6.33 .001 .232
Tukey post-hoc tests revealed that children in the highest ability group of Production at 12 months had significantly higher Comprehension scores at 12 months (between lowest ability group and highest ability group, p < .001; between low-average ability group and highest ability group, p = .003; between high-average ability group and highest ability group, p = .002)43 and 18 months
than the children in any of the other groups and the other three groups did not differ significantly between each other.
Furthermore, children in the highest Production ability group at 12 months continued to have significantly higher Production scores at 18 months and Comprehension scores at 24 months compared to the two lowest groups but the highest ability group was not significantly different from the average-high group. The lowest ability children did not differ significantly from the children in the two average ability groups.
169 Early precocious talkers still had significantly higher scores than children with slow initial development (1st-25th percentile) in terms of Expressive Communication scores six months later (at
18 months), grammar scores one year later (at 24 months) and higher Production scores up to 2 years later (at 24 and 36 months). The average groups did not significantly differ from any of the other groups.
Interestingly, the lowest ability group for Production at 12 months had significantly lower grammar scores at 36 months than all other children. The other three groups did not differ in terms of grammar scores.
We expected that language was more stable at 18 months compared to 12 months, therefore we calculated the same MANOVA with Production ability at 18 months (IV) and the communication variables (IV, see Table 24 except for language variables at 12 months and Production at 18 months) in the next step. Again, a MANOVA was used and the result was significant (Pillai’s trace V = 1.02, F (36,162) = 2.33, p < .001), see Table 26 for the separate ANOVAs.
Table 26. Separate univariate ANOVAs between Production ability at 18 months and the communication variables
Source F Sig.
Partial Eta Squared Levels of ability for
Production 18m Comprehension 18m F(3,63) = 16.88 .000 .446 Gestures 18m F(3,63) = 5.38 .002 .204 PLS Auditory Comprehension 18m F(3,63) = 7.32 .000 .258 PLS Expressive Communication 18m F(3,63) = 25.74 .000 .551 Comprehension 24m F(3,63) = 18.99 .000 .475 Production 24m F(3,63) = 19.49 .000 .481 Sentence Complexity 24m F(3,63) = 17.09 .000 .449 PLS Auditory Comprehension 24m F(3,63) = 2.42 .075 .103 PLS Expressive Communication 24m F(3,63) = 10.33 .000 .330 ASQ communication 36m F(3,63) = 5.06 .003 .194 Production 36m F(3,63) = 7.34 .000 .259 Sentence Complexity 36m F(3,63) = 4.86 .004 .188
170 Separate ANOVAs showed that Production ability at 18 months had a significant effect on most communication variables (except for Gestures at 12 months and Auditory Comprehension at 24 months). The effect sizes were large.
The significant relationships were further analysed using post-hoc Tukey tests. The lowest and highest groups were significantly different from each other in all situations; this means children who were classified as late talkers (production at or below 25th percentile) at 18 months already
showed significantly lower concurrent communication scores and showed significant lower vocabulary, general language and grammar scores up to 18 months later (at 36 months) when compared to those high-achieving children at 18 months.
There was no scenario were all groups differed significantly from each other. In addition, groups were less likely to differ if they were of neighbouring ability at 18 months (e.g. the two middle groups often did not differ significantly from each other). For Production at 18 months the high- level group was significantly different (higher) from the other three groups in terms of Comprehension and Production at 24 months. In contrast, the lowest scoring group was not significantly different from the low-medium group in those two situations.
At 36 months, only the high and low ability group differed significantly from each other in terms of Sentence Complexity. At this point, Production was still the best outcome variable to distinguish the children from each other; however, the low ability group was not significantly different from the low-medium ability group and the high ability group was not different from the high-medium ability group (again the two middle groups did not differ significantly from each other). As expected with regards to the ASQ, the high ability group was not significantly different compared to the middle ability groups (due to the purposefully created negative skew when constructing the tool); however, the low ability group was still of significantly lower ability regarding the middle-high and high ability group but not the low-medium ability group.
171
6.2.2. Comprehension
A MANOVA was used with Comprehension ability at 12 months (IV) and the different communication variables (DV), see Table 24 except for Comprehension at 12 months. The analysis showed that Comprehension ability at 12 months had a significant effect on communication scores between 12 and 36 months (Pillai’s trace V = 1.08, F (45,153) = 1.91, p = .002), see Table 27 for the results of the separate ANOVAs.
Table 27. Separate univariate ANOVAs between comprehension ability at 12 months and the communication variables
Source F Sig.
Partial Eta Squared Levels of ability for
Comprehension at 12m Production 12m F(3,63) = 9.08 .000 .302 Gestures 12m F(3,63) = 2.00 .125 .086 Comprehension 18m F(3,63) = 12.52 .000 .373 Production 18m F(3,63) = 3.85 .014 .155 Gestures 18m F(3,63) = 1.22 .309 .055 PLS Auditory Comprehension 18m F(3,63) = 3.65 .017 .148 PLS Expressive Communication 18m F(3,63) = 2.85 .045 .119 Comprehension 24m F(3,63) = 11.91 .000 .362 Production 24m F(3,63) = 5.41 .002 .205 Sentence Complexity 24m F(3,63) = 1.36 .264 .061 PLS Auditory Comprehension 24m F(3,63) = 1.76 .164 .077 PLS Expressive Communication 24m F(3,63) = 2.72 .052 .115 ASQ Communication 36m F(3,63) = 2.94 .040 .123 Production 36m F(3,63) = 2.91 .041 .122 Sentence Complexity 36m F(3,63) = 1.50 .223 .067
The data indicates that very early Comprehension (12 months) ability had a significant effect on several communication variables up to 36 months showing mostly large effect sizes. There were also some ANOVAs with non-significant results (i.e. Gestures at 12 and 18 months, Sentence Complexity at 24 and 36 months, PLS scores at 24 months).
Post-hoc tests showed that the Comprehension ability group at 12 months had a significant effect on Production scores at 12 months. Children from the highest ability group had significantly higher scores compared to all other three groups which showed lower Production skills and they
172 did not differ from each other. In addition, Comprehension ability at 12 months had a significant effect on Comprehension scores at 18 and 24 months. Whilst the neighbouring scores at the extreme ends (between the low and low-average ability groups well as between the average-high and high ability groups) were not significantly different from each other, all the other groups did significantly differ from each other.
In addition, Comprehension ability at 12 months also had a significant effect on Production scores (18, 24 and 36 months). At 18 months, children from the highest ability group had significantly higher scores compared to the two lowest groups only. The other groups were not significantly different from each other. At 24 months, the low-average ability group showed significantly different scores from the highest and the average-high group. The other groups did not differ significantly which was interesting particularly for the lowest ability group which consistently showed significantly lower scores in terms of Production ability (see MANOVAs above). Similar results were found at 36 months, only the low-average ability group was significantly different from the highest ability group in terms of Production scores and only the two average ability groups differed in terms of ASQ scores, again the other groups were not statistically different from each other.
Another MANOVA was used with Comprehension ability at 18 months (IV) and the communication variables (IV, see Table 24 except for language variables at 12 months and Comprehension at 18 months). The model as a whole had a significant effect on communication scores between 12 and 36 months (Pillai’s trace V = 1.16, F(36,162) = 2.85, p < .001). Furthermore, Comprehension ability at 18 months had a significant effect on all scores except for the ASQ at 36 months (see Table 28 for the results of the separate ANOVAs), this contrasts with Comprehension ability at 12 months (see Appendix 13 for table of results). The effect sizes were large for all significant ANOVAs.
173 Table 28. Separate univariate ANOVAs between Comprehension ability at 18 months and the
communication variables Source F Sig. Partial Eta Squared Levels of ability for Comprehension 18m Production 18m F(3,63) = 12.43 .000 .372 Gestures 18m F(3,63) = 13.73 .000 .395 PLS Auditory Comprehension 18m F(3,63) = 11.95 .000 .363 PLS Expressive Communication 18m F(3,63) = 8.08 .000 .278 Comprehension 24m F(3,63) = 26.92 .000 .562 Production 24m F(3,63) = 14.72 .000 .412 Sentence Complexity 24m F(3,63) = 7.84 .000 .272 PLS Auditory Comprehension 24m F(3,63) = 5.38 .002 .204 PLS Expressive Communication 24m F(3,63) = 5.04 .003 .193 ASQ Communication 36m F(3,63) = 2.45 .072 .104 Production 36m F(3,63) = 12.77 .000 .378 Sentence Complexity 36m F(3,63) = 3.61 .018 .147
Post hoc tests revealed that Comprehension at 18 months had a significant effect on all Gestures, vocabulary (Comprehension and Production), general language scores (PLS-5 UK) and grammar scores (except for the ASQ) in which high ability and low ability groups were significantly different from each other. Communication scores were always significantly lower for the low ability group compared to the high ability group.
Comprehension level (18 months) was best at distinguishing between Production scores (18 months) with the high Comprehension ability group showing significantly higher Production scores than the other three groups. Within these groups, only the lowest and average-high group also differed significantly demonstrating significantly lower Production scores in the lowest Comprehension ability group compared to the average-high group. At 18 months, children in the lowest Comprehension group also had significantly fewer Gestures than all the other groups (the other three groups did not differ significantly). In terms of general language scores on the PLS, significant differences were only found for more distant but rather than neighbouring Comprehension ability groups.
174 Furthermore, Comprehension ability showed significantly different scores in terms of Comprehension scores (at 24 months), whilst the average-high and high ability groups were significantly different from all other groups (and remained in the same order), the two lowest groups were not significantly different from each other. Furthermore, children in the highest Comprehension ability group at 18 months showed superior Production scores at 24 and 36 months, the three lower groups did not differ from each other. Only distant groups showed significant differences in terms of general language scores at 24 months (low and high ability groups, low-average and high ability groups, low and average-high ability groups) and grammar scores at 24 months (low and high ability groups, low-average and high ability groups) and 36 months (low and high ability groups).
6.2.3. Gestures
When running the MANOVA with Gesture level at 12 months or 18 months (IVs) and the communication variables (DVs), the models were not significant (see Appendix 13 for overview of results).
6.3. DISCUSSION
In this analysis, children were allocated into one of four ability groups for Production, Comprehension and Gestures at 12 and 18 months. Four ability groups were created from the