48 Factors associated with differences in digitally measured student perceptions of teaching quality education. Classes normally consist of 25 to 28 students. Mathematics is one of the main subjects in Dutch secondary schools. All teachers were participating in an intervention study (see chapter 4 of this dissertation) in which the Impact! tool was used to collect data about the quality of teachers’ teaching. 3.3.2 Impact! instrument The Impact! tool was originally developed for use by secondary school teachers, who could digitally collect student perceptions of teaching quality right after a lesson. Teachers could use the data obtained as feedback to improve the quality of their teaching in follow-up lessons. A company developed the technical part of the Impact! tool, based on the specifications developed by the author of this dissertation and colleagues. Impact!’s construct validity and reliability were investigated (see chapter 2 of this dissertation) using a combined item response theory (IRT) and generalizability theory (GT) model. The correlations between the characteristics of effective teaching were high (> 0.9), and results showed good model fit with teaching quality as a unidimensional construct. The reliability of the Impact! questionnaire was high, .895 (with an average of 19.07 students per classroom and 6.81 time points at which Impact! was used to measure teaching quality). The tool is now available for primary, secondary, and higher education and mainly used for teacher development and research purposes. The tool is sold by a company. 3.3.3 Measures The Impact! questionnaire was developed in a thorough way; the items were designed based on a literature review of student perception questionnaires and a review of teacher effectiveness research, input from experts in the educational sciences, extensive discussions about draft versions of the questionnaire, and a pilot with teachers and students. The questionnaire reflects teaching characteristics that have been found to be effective for student learning: a supportive and positive classroom, organized and structured classroom management, clear instruction, good quality teacher–student interaction, cognitive activation of students for deep learning, adaptive instruction, and the formative assessment of students’ learning. During a period of 4 months in the 2016–2017 school year, the teachers and their students used the Impact! tool at the end of a number of math lessons chosen by the teachers. In this way, student perceptions of the quality of their mathematics teachers’ teaching were collected. The number of measurement occasions differed between teachers, ranging from 3 to 17, with an average of 7.
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