121 Short-term outcomes and six-month follow-up of BEAR 6 joint engagement, coordinated joint engagement and symbol-infused joint engagement. The four items were each defined with 7 points providing information about both quantity and quality of engagement states. The low anchor (1) indicates that there are no episodes of joint engagement during the interaction, whereas the high anchor (7) indicates that the child almost always spends time in the joint engagement state during interaction. The midpoint of 4 characterizes a child who is in joint engagement for approximately one half (total joint engagement item) or one third (supported joint engagement, coordinated joint engagement and symbol-infused joint engagement items) of the scene. The videos were coded using the JERI by a team consisting of six psychology master students and one research assistant. Coders were trained to use the JERI coding scheme reliably, before they independently coded clips by the first author MS (who was trained up to reliability by one of the developers of the JERI). Coders were unaware of treatment allocation. To ensure reliable scoring throughout, booster meetings were held in which coders received feedback from the trainer (MS). Booster session scores were included in rating inter-rater reliability. In total, 15% of the videos were double coded. Inter-rater reliability varied from κ = .59 to κ = .81, indicating a moderate to excellent reliability (see Table 2). Secondary Outcome Measures Child Outcomes Brief Observation of Social Communication Change (BOSCC). Social-communicative development was also examined at T1, T2 and T3 through administering the BOSCC (Grzadzinski et al., 2016). The BOSCC was rated based on the same 12-minute parent-child dyadic interaction as the primary outcome measure. The BOSCC consists of 15 coding items associated with key features of ASD, such as making eye contact, unusual sensory interests and the frequency and function of social overtures. These items were rated from 0 to 5 points. The change in total score is indicative for change in behavior in the socialcommunicative domain. The second author CG trained with MS and IO in administering the BOSCC to achieve research reliability. Subsequently, a team consisting of one research assistant and two master students who were masked to treatment allocation were trained by CG and were deemed fully trained when coders were within 1-point difference for ≥80% of items on a segment, and when total change scores per segment were within 3 points for three consecutive video’s (Grzadzinski et al., 2016). Double coding was completed by CG for every 5th video to ensure inter-rater agreement. Autism Diagnostic Observation Schedule- Second Edition (ADOS-2). Severity of ASD related symptoms were assessed at both baseline (T1) and follow-up (T3) with use of the Dutch translation of the ADOS-2 (De Bildt et al., 2011). Depending on the age and expressive language level of the participants, either the toddler module, module 1 or module 2 was administered. In research, calibrated severity scores (CSS) are used as a metric for severity
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