90 Factors influencing teachers’ use of digital student feedback to improve their teaching months to obtain digital feedback from students (students were all 14-15 years old) about the quality of their teaching after some of their lessons. The tool was used by every teacher at least three times in their lessons (they chose the lesson they wanted to receive feedback on). The teachers could use the feedback obtained by means of the Impact! tool as data to improve the quality of their teaching in follow-up lessons. Teachers received only an introduction to the use of the Impact! tool; no additional training on how to use the digital feedback data to improve their teaching was provided. To answer our research question, f irst, interviews were conducted with all teachers and students involved in the study (Strauss & Corbin, 2015), to study the factors perceived by teachers and students as influencing the use of digital student feedback. Second, a comparative case study was conducted (Bartlett & Vavrus, 2017), to compare two teachers and their students with regard to their perceptions: one teacher who improved their teaching based on the digital student feedback and one teacher who did not. 5.3.1 Sample interviews Nine teachers and 23 of their students (2-3 per teacher) were invited to participate in the study, based on purposive sampling (Patton, 2002; Robinson, 2014). This is a non-random sampling method in which participants are deliberately approached based on several criteria. Teachers were chosen based on the number of times they used the Impact! tool, their opinions about the smartphone-assisted student feedback (based on teachers’ own general reports of their thoughts of the potential improvement value of digital student feedback, as measured on the posttest), and teachers’ changes in teaching quality scores between the pretest and posttest (the pretest and posttest are described in the following section). Students were selected based on the number of times they gave digital feedback to their teacher, their opinion about the smartphone-assisted student feedback (based on students’ own reports about the use of the smartphone application) and their level of performance (based on their mathematics grade). For each of these criteria, we strived for high variation with regard to a) the number of times teachers and students used the Impact! tool, b) teachers’ opinions about the potential impact the tool has, c) the changes in teachers’ teaching quality after the student feedback intervention and d) students’ opinions about the digital student feedback. One teacher and one student did not want to be interviewed, and one student
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