15 1 To f ind statistical support for the construct validity of the Impact! questionnaire, the f it of the combined IRT and GT model was investigated. The model also made it possible to simultaneously estimate variance components (students, teachers, times, and their interactions), to determine the reliability of the scores from students. In addition, a decision study (d-study) was conducted to investigate what happens with the reliability of the measurements in the case of more or fewer measurement times and students. 1.6.2 Factors associated with differences in student perceptions of teaching quality Insight into confounding variables involved in student perceptions of teaching quality can help us understand better what is measured when we collect student perceptions of teaching quality. Our innovative statistical methods from the first study (section 1.6.1) could be used to scrutinize the specific effects of several variables, as was demonstrated when investigating the following research question: What factors on the student, teacher and class levels are related to differences in student perceptions of teaching quality? To answer this question, we used the same data and the same full model as in the first study. Theoretically grounded variables that are potentially associated with differences in student perceptions of teaching quality were added as covariates to the model. These variables were teachers’ age, gender, teaching experience and popularity among students, students’ gender, their performance level and attitude towards the feedback, class size and the average performance level of the students in a class. Factors that proved to be related and unrelated to differences in student ratings were reported and discussed, as well as the advantages of the complex statistical multi-level approach for evaluating extensive longitudinal designs and for giving insight into the factors biasing student perceptions of teaching quality. 1.6.3 The effectiveness of student feedback In this study, we investigated what happens when the student teaching quality ratings are provided to teachers as feedback on their instruction, in order to develop their insight into the strengths and weaknesses of their lessons. The goal was to promote teachers’ professional reflection on the quality of their teaching as a basis for their attempts to improve their teaching. The following research question was answered in the study: Does student feedback promote
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