**4. Explorative investigations on data of a Swiss PR study2**

The investigated data are derived from an extracurricular PR study with third grade students (*N* = 198) conducted in Switzerland from 2014 to 2015. The target group consisted of students struggling with reading fluency who were determined as "in need for training" by means of a standardized screening test and by their teachers' perception. A randomized control field trial with two experimental groups (parent tutors and volunteer tutors) was conducted (pretest, posttest, and follow-up). The findings showed that children who trained with volunteer tutors developed significantly better reading fluency after 20 weeks compared to the children with parent tutors and control group [10]. Great efforts were put into checking treatment fidelity, by collecting self-reported data (questionnaires and record books), and observational data (video-taped). Most of the participants conducted the training as intended. However, the variability of implementation fidelity among the participants was rather high, also among parent tutors [42]. Moreover, some children with parent tutors still showed high gain in reading fluency. Therefore, the present investigations focus on possible differences between students within this condition (parent tutor) with very low and very high gain in reading fluency. The following research questions are addressed:

