122 Chapter 6 that is less influenced by intellectual ability, language and age (Gotham et al., 2009). However, for the Dutch translation, the CSS was created for modules 1 through 4 but not for the toddler module (de Bildt et al., 2011). Therefore, algorithm total scores were calculated and compared, with higher scores indicating more severe ASD symptoms to ensure comparability across modules. It is important to mention that the algorithm total scores are more likely to be influenced by verbal language levels, meaning that scores on the Toddler Module are likely to be higher for children with significant language delays (Esler et al., 2015). Nevertheless, total scores (combined social affect and rigid and repetitive behaviors scores) allowed for meaningful comparisons by providing a continuous measure of emerging ASD symptomatology (Green et al., 2017). Language outcomes. Language comprehension and language production were measured through parent-report by the Dutch adaptation of the MacArthur Communicative Development Inventory: Toddler (N-CDI; Fenson et al., 1993, 2000) at T1 and T3. Furthermore, language production was measured by professional observation using the JERI. Based on the same 12-minute dyadic videotape child’s expressive language level and use were rated from 1 (no expressive language) to 7 (fluent and frequent use of sentences) at T1, T2 and T3. Vineland Screener. To estimate the global level of adaptive functioning, the Vineland Screener was conducted at T1 and T3. The Vineland Screener is an adapted Dutch research version of the Vineland Adaptive Behaviour Scales (VABS: Sparrow et al., 2005). The Vineland Screener provides parent reported information about the child’s everyday adaptive skills on the following domains: Communication, Daily Skills, Socialization and Motor Skills. The overall adaptive behavior composite (ABC) standardized scores were used, with higher scores indicating better adaptive skills. Parent Outcomes Parental skills. Parental skills were rated by the parent scales of the JERI and consist of four items: following in on the child’s focus, scaffolding, symbol highlighting and caregivers’ affect (see Adamson & Suma, 2020 and our protocol paper Snijder et al., 2022 for a detailed description). Just as with the joint engagement items, parent scale items were rated from a scale of 1 to 7. Parental skills were rated based on the same videotaped dyadic interaction at T1, T2 and T3 as the joint engagement measure and was rated by the same coding team. Interrater variability of the parental intervention skills varied from κ = .57 to κ = .73, indicating a moderate to substantial inter-rating variability. Parental well-being. Parental well-being was measured by the Warwick-Edinburgh Mental Well-being Scale (WEMWBS; Tennant et al.,, 2007) and the Parenting Stress Questionnaire (PSQ, in Dutch Opvoedingsbelastingsvragenlijst, Vermulst et al., 2012). The WEMWBS
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