Lengthy studies in Texas and Tennessee have shown that students taught by fully certified and experienced Texas Teachers can overcome significant economic and environmental challenges to succeed. The likelihood of success for the same students with poorer teachers is much less. The Tennessee study found that high-achieving students with the most effective Texas Teachers gained 25 percentile points on standardized exams in one year, while those with the least effective teachers gained only two points. The impact on low-achieving students was even greater; those placed with the most effective teachers gained 50 percentile points, while those with the least effective teachers gained only 14 points. Texas researchers studied fourth-graders in the Dallas ISD for three years in a row, contrasting those who had effective Texas Teachers, as measured by previous student performance on achievement tests, with those judged to have ineffective teachers. The scores of the fourth-graders with effective Texas teachers rose 16 percentile points in reading and math by sixth grade, while the scores for students assigned to ineffective teachers dropped by 18 percentile points in reading and 33 points in math. There appears to be a direct correlation between stable and qualified Texas Teachers and test scores at campuses experiencing high turnover and those that assign teachers to subjects outside their fields of study.