There are several limitations for the results of the present thesis. Some of the limitations apply to the whole thesis; others are directly related to particular articles or the additional analysis in Appendixes 4.
Regarding limitations that apply to the whole thesis, the operationalization of the term bilingual learners is perhaps the most prominent one. Bilingualism has in all three articles been treated as a categorical term. There are some attempts to test different dimensions of bilingual experience in Study 1, but the dimensions are tested individually and not as a combination of different dimensions as a true continuous dimension (e.g., Bialystok & Barac, 2012; De Cat, 2020; Luk
& Bialystok, 2013). Studies 2 and 3 relied on comparisons of different bilingual subgroups against a monolingual control group. Such comparisons could be problematic since it is difficult to design non-overlapping groups (Genesee, 2010; Luk & Bialystok, 2013). Hence, the use of bilingualism as a continuous variable could have led to the creation of different and more suitable subgroups of bilingual learners.
It would also have been possible to use the continuous variable in a regression to determine how the degree of bilingualism was related to skills in a combined group of monolingual and bilingual learners. Thus, if bilingualism were measured as a continuous variable, the conclusions regarding the examined hypotheses of the present thesis could have differed.
Since bilingualism was treated as a categorical variable in this thesis, more characteristics of the samples in Studies 2 and 3 would have been preferable. As mentioned in the discussion of the results of Study 2, more knowledge of the bilingual parents’ nationality would have strengthened the paper. In the discussion of Studies 2 and 3 in this thesis, I have assumed that bilingual learners with a native majority language-speaking parent growing up in Norway, with both languages being used at home
in toddlerhood, are bilingual first learners. It would have improved the thesis if we had indeed asked the parents about the precise AoA of their bilingual child. The method of sample selection in The Stavanger Project also limits the conclusions of the hypotheses examined in this thesis. The Stavanger Project was designed to examine the development of skills such as social functioning reading and language in all children. Hence, there were no exclusion criteria for children displaying atypical development. This could be a validity challenge to the group comparison studies conducted in articles 2 and 3. One way to reduce this threat would have been to ensure that the compared groups were equal on all other third factors that could influence the examined factor (in this case language level). Hence, group performance across the examined groups is more likely caused by the grouping factor (here monolingual or bilingual status) than by a hidden third factor. The lack of exclusion of children with atypical development in The Stavanger Project is therefore not optimal. There is a chance that children with atypical development are unevenly distributed across the different groups examined in the different articles. An uneven distribution of potential learning disability across groups could then work as a hidden third factor influencing the language levels of the monolingual or bilingual group beyond what can be expected on the basis of monolingual and bilingual status. This is particularly problematic for the results of Study 2 and the analysis in Appendix 4. In Study 3, the children were older; hence, signs of learning disabilities would most likely have emerged. When invitations to participate in Study 3 were sent, some parents returned a consent form stating that their child unfortunately could not take part in it. The parents then explicitly explained that although they valued the aims of the project, their child was not a good candidate for the project since the child had been diagnosed with learning disabilities. The child would therefore not be representative of typical developed monolingual or bilingual children. It is not clear whether all parents of children with learning disabilities provided such information. The type of assessment used in Study 3 provided, however, additional support in excluding
children who had obvious learning disabilities. The language and reading comprehension levels of all participants were individually evaluated by teachers with long experience with children. The completion of the individual tests took 1 hour; hence, it was possible to detect age-inappropriate behaviour. In one case, a bilingual child was unable to finish the tests, and his test scores were then excluded from the study.
Even though such an evaluation of the children was performed, it is important to acknowledge that the parents of participants in Study 3 were not specifically asked whether their child had learning disabilities. Thus, there is still a possibility that an uneven distribution of learning disability between groups influenced the results of the analysis in Study 3.
Another important limitation is the lack of a systematic search for relevant articles regarding the hypothesis of poorer language levels. Such a search, followed by analysis of the data in a meta-analysis, would have strengthened the quality of this thesis. Even though the time spent searching for relevant articles was extensive (articles about pre-adolescent minority language learners with 4-7 years of L2 exposure, pre-adolescent early bilingual learners, pre-adolescent bilingual first learners), a systematic search could have identified a range of studies that contradict the results of the identified studies and the present analysis. Hence, in coding and analysing all relevant studies, the conclusions regarding these hypotheses could potentially differ from what was possible based on a narrative review. Furthermore, a meta-analytic approach would have enabled us to form a conclusion regarding to what extent the levels of reading comprehension and language of early bilingual learners resemble the levels of minority language learners, or their monolingual peers. Casting light on this problem is beyond the scope of for the present approach.
Regarding the measurement used to examine language comprehension in Study 2, the results could have been hampered by comparing groups across sum scores. It would have strengthened the validity of the results if data from all the TRAS sections were examined by explorative and
confirmatory factor analysis, followed by securing an invariant construct across the bilingual and monolingual groups. Such an approach, instead of a theory-driven approach, would have allowed for comparisons of true scores. As it stands now, one cannot be sure how much of the observed differences across language groups could be ascribed to comparisons across different constructs, or even whether some items in the measurement instrument are biased for one language group despite otherwise equal language levels. The true differences in levels of language comprehension might therefore be either larger or smaller than the observed differences identified in article 2.
For Study 3, as with many other invariance studies, there is a limitation regarding these analysis in the article. The chosen procedure in article 3 relies on an assumption of a normal distributed latent variable underlying the ordered categorical items. Weather these variables in study 3 indeed are normally distributed is however not empirically investigated. If the latent response variates that underly the categorical variables are not multivariate normal, invariance in the thresholds and factor model parameters cannot guaranty measurement invariants for the measured variables (Millsap & Yun- Tein 2010). Another limitation is the sample size in this article. A larger sample size would have been preferable for many reasons. This would have allowed a more complicated model, either testing the dimensions in constructs or examining a bifactor model.
These more complicated models would have enabled an examination of the individual contributions of different linguistic constructs to reading comprehension, controlled for the common underlying language ability.
Such knowledge would be preferable when designing intervention studies for weak monolingual and bilingual readers.
Finally, there is a limitation regarding how well the different papers address the overarching hypothesis of poorer language levels in bilingual than in monolingual learners. The articles themselves could have provided a better examination of how well poorer language levels can be generalized across different groups of bilingual learners. In hindsight, it
would have been preferable for article 2, research question 2 to compare the different bilingual subgroups to the monolingual control group instead of solely comparing the levels of the three different bilingual subgroups. The reason for this limitation is that at the stage in the process where article 2 took shape, the overarching aims of the thesis were not fully developed. The overarching aims thus could not work as guidance for the formation of article 2. (Despite being labelled article 2, it was the first completed article of the thesis.) Thus, in order to fully address this overarching hypothesis, in article 2, the language levels of a composite group of bilingual learners exposed to some or mainly L2 at home are compared to the language levels of monolingual learners. The results of this analysis are presented in Appendix 4.