Receptive and Expressive Vocabulary Breadth. In terms of both receptive and expressive vocab- ulary breadth knowledge, both groups of children made a significant rate of progress over time. The average absolute raw increase across both groups in correctly identified words between each time point was higher for receptive (7.66) than expressive (2.49) breadth, reflecting the different ranges and maximum scores of the measures employed (i.e. BPVS and CELF EV, respectively). Reference to the summary of intercepts and slopes in Table 4.21 indicates that despite their lower starting point at t1, EAL learners made a slightly faster rate of progress than their monolingual peers on both measures across the course of the study (receptive slopes = Mono: 7.47; EAL: 7.75; expressive slopes = Mono: 2.28; EAL: 2.64). This slightly faster rate of progress, however,
interaction term (see Table 4.10). There was a degree of nonlinearity in the data: performance on receptive vocabulary breadth (BPVS-III) for both groups accelerated between t1 and t2, before decelerating by t3. For expressive breadth performance, this same pattern of acceleration and deceleration was found again for the monolingual group but not the EAL group, which showed a fairly constant rate of acceleration across all time points. Nevertheless, in both cases the magni- tude of group differences at t3 for receptive and expressive breadth had decreased to below that found at t1, suggesting some convergence between the groups.
Although there is comparatively little longitudinal research on vocabulary development in EAL learners, the findings from the present study do compare closely with others in the literature. The results of Hutchinson et al. (2003) and Burgoyne et al. (2011) bear very close similarity to the trajectories described above for receptive and expressive vocabulary breadth. Although these studies recruited and followed children at earlier developmental stages (Years 2 and 4; ages 6 to 9), results showed that EAL and monolingual groups made a very similar rate of progress across time in both receptive and expressive breadth knowledge, such that monolingual advantages at t1 remained by t3. Interestingly, results of the present study also show a deceleration of the mono- lingual group in receptive breadth knowledge between the penultimate and final time point, but do not provide evidence of a significantly faster rate of growth of EAL learners in vocabulary knowl- edge. On the other hand, there are longitudinal studies which indicate a significantly faster rate of vocabulary development of bilingual learners: particularly, bilingual children followed between Grades 1-6 in Farnia and Geva (2011) in Canada, and between Grades 2-4 in Simos et al. (2014) in Greece. Additionally, both studies also found evidence of nonlinearity in this development due to faster rates of growth in earlier grades, followed by deceleration in later ones. This is supported to some extent by the results of the present study, although more time points would provide a more robust comparison (cf. 6 in Farnia & Geva and 5 in Simos et al).
Expressive Vocabulary Depth. Similar to trajectories for vocabulary breadth measures, both groups of children made significant progress over time in expressive vocabulary depth knowledge, scoring an additional average of 2.08 raw points at each subsequent time point. Reference to Ta- ble 4.21 shows that the EAL group made a faster rate of progress over time than the monolingual group in WISC VC performance (Slopes = Mono: 1.84; EAL: 2.29), although not significantly so, as indicated by the lack of a significant time × group interaction term. However, the combination of a lower intercept at t1 and a faster rate of development resulted in a closing of the gap by t3, at which point the effect size of the group difference, although negligible, had reversed in the direction of an EAL group advantage (d = -0.03).
Studies of early language development show that both monolingual and bilingual learners make progress over time in vocabulary depth knowledge, and there is some evidence to sug- gest that the trajectories of the two groups are similar (Hadley et al., 2016; Karlsen et al., 2017). Karlsen et al. (2017) assessed a sample of 191 monolingual Norwegian and 66 bilingual Urdu / Punjabi-Norwegian 5 year-olds on measures of receptive vocabulary breadth and depth at the end of kindergarten and again in the first year of school. Results showed that although both groups of children made progress in their vocabulary knowledge over time, they did not differ sig- nificantly from one another in the rate of this progress, despite the bilingual group beginning on a lower intercept on both measures. Interestingly, however, and in accordance with results from the present study, the two groups did converge slightly over time in vocabulary depth knowledge
from d = 1.35 to 1.06. Finally, more evidence of the similarity between mono- and bilingual learn- ers in vocabulary depth trajectories comes from intervention studies showing that both groups of children benefit equally from vocabulary instruction, discussed further in Chapter 5 (e.g. Carlo et al., 2004; Silverman, 2007).
Summary. Research question 1 asked to what extent the two groups differed in their per- formance at t1. Although the monolingual group exhibited advantages on all three measures of vocabulary knowledge, group differences were significant only for measures of breadth and not depth of knowledge. Effect sizes of group differences in all vocabulary variables were smaller than those commonly reported in the literature, especially for vocabulary depth, and it should be noted again that the two groups did not differ to a large extent in terms of absolute number of words correctly identified. The lack of a significant group difference in vocabulary depth is supported by an alternative analysis employing a bespoke scoring rubric, which indicated that children in both groups tended to provide the same kinds of answers when asked to give a definition, with the exception of the monolingual group producing significantly more answers relating to background knowledge.
Research question 2 asked to what extent the developmental trajectories of the two groups were comparable across the course of the study. Results showed that EAL learners made a con- sistently faster rate of progress over time than their monolingual peers on all vocabulary measures but that, where t1 group differences were significant (i.e. in receptive and expressive breadth), this faster rate of progress was not sufficient to close the gap entirely by t3. In contrast, the mono- lingual advantage in vocabulary depth at t1 was no longer present at t3. One other interesting observation was that while the monolingual group tended to decelerate between t2 and t3 on some vocabulary measures, the EAL group was more likely to maintain a similar trajectory throughout all time points. In sum, these findings suggest that even at t1, the two groups of learners were already performing more similarly to each other than what is reported in previous studies, and that convergence between the two groups in receptive and expressive vocabulary breadth did not take place to any significant degree.