While there has been considerable anecdotal discussion about the importance of school principals, there has been very little systematic research into their impact on student outcomes. Recent analysis in the United States has examined how the gains in student achievement at a school change after the principal changes. This outcome-based approach to measuring effectiveness of principals is very similar to the value-added modeling that has been applied to the evaluation of teachers. Such research in the state of Texas found that principals have a very large impact on student achievement. Effective school principals have been shown to significantly improve the performance of all students at the school, at least in part through their impacts on selection and retention of good teachers. Ineffective principals have a similarly large negative effect on school performance, suggesting that issues of evaluation are as important for school administrators as they are for teachers. The impact of principals has also been measured in non-traditional ways. [clarification needed] Some principals have focused their efforts on creating more inclusive schools for students with disabilities.