Schools can vary on overall school achievement (proficiency) as well as the amount of growth in student learning, For example schools A, B, and C all have high achievement levels but only in School C do students have, on average, high growth. Schools D, C, and F all have low levels of proficiency but only in school D do students, on average, have low growth.
Growth models have intuitive appeal to teachers as they focus on how much a student learned during the school year not what the student knew at the start of the school year. The current research evidence suggests that teachers matter a lot i.e. students learn much more with some teachers than others. For example, in one study low-achieving fourth grade students in Dallas, Texas were followed for three years and 90 per cent of those who had effective teachers passed the seventh grade math test whereas only 42 per cent of those with ineffective teachers passed (cited in Bracey, 2004). Unfortunately, the same study reported that low achieving students were more likely to be assigned to ineffective teachers for three years in a row than high achieving students. Some policy makers believe that teachers who are highly effective should receive rewards including higher salaries or bonuses and that a primary criterion of effectiveness is assessed by growth models, i.e. how much students learn during a year (Hershberg, 2004). However, using growth data to make decisions about teachers is controversial as there is much more statistical uncertainty when using growth measures for a small group or students (e.g. one teacher's students) than larger groups (e.g. all fourth graders in a school district).
Growth models are also used to provide information about the patterns of growth among subgroups of students that may arise from the instructional focus of the teachers. For example, it may be that highest performing students in the classroom gain the most and the lowest performing students gain the least. This suggests that the teacher is focusing on the high achieving students and giving less attention to low achieving students. In contrast, it may be the highest performing students gain the least and the low performing students grow the most suggesting the teacher focuses on the low performing students and paying little attention to the high performing students. If the teacher focuses on the students “in the middle” they may grow the most and the highest and lowest performing students grow the least. Proponents of the value-added or growth models argue that teachers can use this information to help them make informed decisions about their teaching (Hershberg, 2004).
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