2.7. Fidelity of Implementation Hastie and Casey [54] suggested that researchers should provide: (a) a rich description of the curricular elements of the unit, (b) a detailed validation of the model implementation based on the pedagogical model, and (c) a detailed description of the programme context to help the reader understand the research design and the results obtained. Elements (a) and (c) have already been described in the teacher specific professional development section. For the detailed validation of the model implementation, the research group filmed one in every six sessions (5 sessions, 275 min), which was analysed by external observers (5 sessions per teacher divided into 11 observation periods of 5 min). The camera was installed in the classroom six sessions prior to the beginning of the study to allow students to familiarise with it and to prevent non-spontaneous behaviours. Every teacher’s behaviours were evaluated though TARE instrument (described in the instrument section). At the end of every session, the teacher also assessed their own behaviour using TARE [52], in order to encourage reflection on the TPSR programme implementation. The observational analysis was conducted by two university students (not related with the study) with knowledge in the education research field. The observers were trained using the following sequence established by Wright and Craig [52]. First,explanationandclarificationofthemeaningofeachofthecategoriesofthetool(theywereputin different situation examples to distinguish them clearly). Second, the observers together visualised two completed classes implementing the TPSR (corresponding to two lessons applied in different schoolnotrelatedtothepresentstudy)usingTARE2.0. Third,theresultsoftheobserverswereshared to unify criteria. Fourth, when observers obtained an inter-reliability upper of 80%, that meant the reliability was guaranteed, observers were ready to start the analysis of the study sessions. Total agreement (TA) was calculated through the formula: number of total agreements (NTA) divided by agreements (A) plus disagreements (D) (TA = NTA/A + D) [55].