The data in this repository were collected in France, the Netherlands and the UK between 2020 and 2022 to inform the validation of the Q-BEx questionnaire (). Children between the ages of 5 and 9 were tested individually to assess their proficiency in the societal language (i.e., French, Dutch or English), as well as their non-verbal intelligence and working memory. One of their parents or caregivers also filled in the full version of the Q-BEx questionnaire. The respository includes data from 299 children (FR: n=78, NL: n=117, UK: n=104), although some measures are not available for all children. In France, children were recruited in ordinary schools and in private clinics for Speech & Language Therapy (37 children were recruited via clinics). The consent form for schools asked parents if the child had previous or current SLT, and the reason. In the Netherlands, recruitment took place in schools and via social media advertisement. Language disorder (reported by parent/teacher/remedial teacher) was an exclusionary criterion. In the UK, all children were recruited in schools; no exclusionary criteria were applied, and not SLT information was collected. Language experience data Language experience data was collected using the full version of the Q-BEx questionnaire (), which was completed by one of the child’s parents or caregivers. This includes all the following modules (except that Language mixing wasn’t included in France): - Background (languages the child is exposed to, adults and children the child lives with - Risk factors (early language milestones, early parental concerns) - Language exposure and use (current and cumulative estimates; onset of exposure to each language) - Estimates of proficiency in each language (listening, speaking, reading, writing) - Richness of experience in each language (activities, diversity of interlocutors, parental education) - Language mixing - Attitudes The questionnaire was administered either in the societal language (French, Dutch or English) or in one of the child’s home languages (Arabic, Dutch, English, French, German, Italian, Polish, Romanian, Russian, Spanish, Turkish). The choice of administration language was constrained by the translated versions available at the time of testing. The translation protocol used to create the versions in different languages can be found at . Direct outcome measures We collected measures of language and cognitive outcomes during individual, face-to-face sessions with each child (two sessions per child, lasting approximatively 45 minutes each). Most of the testing was done in the child’s school. In France, the children recruited via speech & language therapy clinics were tested in the clinic. In the UK and the NL, some testing sessions took place in a different location (e.g., university premises), and on rare occasions online via Zoom. Language proficiency Outcomes in the societal language (i.e., Dutch, English, or French) include phonology, morphosyntax and vocabulary. Phonology Phonological competence was assessed with the LITMUS Quasi-Universal Non-Word Repetition task. See dos Santos, C., and Ferré, S. (2016) “A Nonword Repetition Task to Assess Bilingual Children’s Phonology”. Language Acquisition 41: 1–14. Morphosyntax Morphosyntax outcomes were assessed with the LITMUS test in each societal language. See Marinis, T. and S. Armon-Lotem (2015). “Sentence Repetition. Methods for assessing multilingual children: disentangling bilingualism from Language Impairment.” in S. Armon-Lotem, J. de Jong and N. Meir, Methods for assessing multilingual children: disentangling bilingualism from Language Impairment. Amsterdam: Multilingual Matters). The English and Dutch versions included 30 items. The French version included 16 items. We created two blocks in the English and Dutch versions so that the first block be comparable to the 16-item French version. Subsequent analyses demonstrated that there was no block effect in EN and NL. For each child, we report three overall scores: Identical Repetitions, Target Repetitions, and Grammatical Attempts. These overall scores correspond to the mean across items (n=30 in English and Dutch; n=16 in French), excluding NAs (i.e., the mean is calculated on the items for which the child did provide a response). Vocabulary Vocabulary breadth was measured in the UK with the British Picture Vocabulary Scale (BPVS), in France with the the Échelle de vocabulaire en images Peabody (EVIP), and in the Netherlands with the Peabody Picture Vocabulary Test (PPVT). Vocabulary depth was measured with the Word Classes component of the Clinical Evaluation of Language Fundamentals CELF-V (in its Dutch, English, or French version). Cognitive measures The tasks used to evaluate cognitive skills were administered in the child’s societal language. Memory Short-term memory was assessed through Forward Digit Recall; working memory was assessed through Backward Digit Recall. Most children were tested using the digit span protocols described in Hill et al (2021), implemented in Psychopy (to allow randomisation of the digit sequences and facilitate the acquisition of detailed data). Children were presented with sequences of numbers (through headphones) and asked to repeat these numbers either in the same order (in the FDR task) or in reverse order (in the BDR task). The length of the sequence increased by one digit after 4 trials, starting with 3 digits in the first block of the FDR task, and 2 digits in the first block of the BDR task. The maximum sequence length was 6 digits in the FDR task, and 5 digits in the BDR task. Children recruited via the Speech and Language Therapy clinics in France (n=37) experienced more difficulty with this task, so it was decided to use the WISC-V protocol instead for these children, as it included a discontinuation rule. To allow comparison across groups, we created a WISC-like score for the data collected via the Hill et al (2021) protocol. This consisted in the digit span for which the child had at least one fully accurate response (i.e., all the digits in the span, in the right order) for at least one out of the first two trials for that span (as the WISC-V protocol only features 2 trials per digit span). FDR_overall and BDR_overall correspond to the total accuracy scores, as per the Hill et al (2021) protocol. FDS_Q and BDS_Q correspond to either the WISC-V score (for children recruited via clinics) or the WISC-like score created as explained above, depending on which protocol the child was tested with. All children have a FDR_Q and a BDR_Q score in the dataset. Non-verbal intelligence The matrices task from either the Wechsler Intelligence Scale for Children–Fifth Edition (WISC–V) or the Wechsler Preschool & Primary Scale of Intelligence - Fourth UK Edition (WPPSI-IV) was used to measure non-verbal intelligence - depending on the age of the child: children below the age of 6 were tested with the WPPSI. Depending on the age of the child at the time of testing, we used the WPPSI protocol (for children younger than 6 years of age, n= 77) or the WISC-V protocol (for all the other children).At least a fifth of primary school children in the UK are bilingual: they grow up with two or more languages. Bilingualism is a linguistic, cultural and economic asset, but it also poses practical challenges for professionals having to deal with an extremely diverse group of children. For instance, it is difficult to evaluate whether a bilingual child's progress in the school language is due to limited language experience (e.g. at home) compared with monolingual peers, or whether it indicates a disorder. Teachers need to know when they can expect bilingual children to have "caught up" with monolinguals in order to inform their assessment of their progress. One of the strongest determinants of children's language development is the amount of time they spend hearing and speaking the language. Measuring this language experience is therefore essential to inform the expectations of teachers and speech and language therapists (SLTs) regarding children's knowledge of the school language. For example, a child who is only exposed to English at school is likely to be less proficient than a child who also hears English from a parent or siblings. This can profoundly affect many aspects of that child's educational development. Researchers have recently started to address the need to quantify the bilingual language experience through the creation of parental questionnaires about the languages spoken to and by the child with different people (parents, siblings, etc.), in different settings (home, school, etc.), and during different activities (reading, watching TV, etc.). Because the questionnaires have been developed by independent teams of researchers, the information they elicit varies - sometimes substantially, resulting in a lack of comparability across studies. Some questionnaires have been used exclusively for research purposes and their length and complexity make them difficult to use by teachers and SLTs, who are often pressed for time and may deal with parents with low levels of proficiency in the community language. Another major challenge is the typically low completion rates of parental questionnaires. Q-BEx will bring a step-change in the measurement of bilingual language experience. * It is the first, multidisciplinary endeavour to establish an optimal metric of bilingual language experience based on a consensus among researchers, SLTs and educators on what aspects of language experience to index, as well as an in-depth review of existing tools. * It will for the first time deliver user-friendly, online questionnaires (and their associated back-end calculators) to return measures of current and cumulative language experience in real time. The questionnaires will be available in 13 languages, and vary in length and level of detail: the shortest version will be useful when parental consultation is challenging; the longest version will yield more fine-grained measures to enable SLTs and researchers to carry out in-depth enquiries. * Q-BEx will be the first to exploit advanced quantitative methods to identify the right level of questionnaire detail (to meet the needs of end-users) and to assess the reliability of the resulting measures as predictors of language proficiency. Reliability and cross-language validity will be assessed using new data from 300 children in 3 different countries. Based on this assessment, Q-BEx will provide evidence-based guidance to inform users' choice on the level of questionnaire detail most appropriate to their needs. * Q-BEx will develop an objective method to identify early those bilingual children in need of support with their school language, thereby addressing the "catch up" question. * Q-BEx will contribute to advancing bilingualism research by exploiting cutting-edge statistical techniques, and making all scripts available to facilitate replication.

The data collection involved recruiting children from schools and private clinics in France, the Netherlands, and the UK. In France, 37 children were recruited via Speech and Language Therapy (SLT) clinics, with consent forms collecting information on any previous or current SLT involvement. In the Netherlands, children were recruited through schools and social media, with language disorders as an exclusionary criterion. In the UK, recruitment was solely through schools without any exclusionary criteria. Language experience data was collected using the Q-BEx questionnaire, covering various aspects of the child’s language background, exposure, and proficiency. Direct language and cognitive measures were collected through face-to-face testing sessions, assessing phonology, morphosyntax, vocabulary, memory, and non-verbal intelligence using established protocols and tests.