others in attracting teachers with strong cognitive skills, so such students are likely to be at a significant disadvantage relative to other students.

A recent ongoing study based on an unusually rich Texas database, containing micro panel data on more than 1.8 million children in five student cohorts attending more than 4,500 elementary schools, provides compelling new evidence of the challenge such schools face with respect to teachers. Kain and Singleton (1996) document that teachers employed in schools with high fractions of disadvantaged minority students have lower ability (as measured by verbal and written test scores on a state teachers' exam), fewer years of education, less experience, and more students in their classes than do teachers in schools with larger percentages of higher-income and white students.

Ferguson's Texas study also points out that the effect of teacher quality is cumulative. Among Texas districts in which students performed poorly in the early years of elementary school, those districts employing teachers with unusually high test scores saw student performance levels in mathematics converging by 11th grade with the levels of districts whose students initially performed well. The scores of children whose teachers had low test scores also converged over the course of their education, but at a much lower level.

While Ferguson's findings are plausible, the issue of causality arises here, as it does in much other research on education. Perhaps there were distinctive characteristics of the districts that were able to retain teachers with high test scores (for example, unusually gifted school administrators or parents with a particularly high degree of commitment to educational quality). Those same factors may have also been responsible for the better performance of students in those districts. Thus, while Ferguson's results are plausible, there is a need for further research that addresses the causality issue.

So how does one promote greater investment in teacher quality in schools serving disadvantaged students? Clearly some special efforts will be needed. Any general policy, such as increasing teacher salaries across the board, may well exacerbate the problems of schools serving disadvantaged students, as the wealthier school districts are in a better position than the poor ones to pay the higher salaries. Moreover, other policies such as tough teacher testing could well decrease the supply of teachers available to teach in the schools with the harshest teaching environments, although Ferguson (1998) found in Alabama that testing new teachers narrowed the gap in basic skills between incoming black and white teachers, to the advantage of black students who are more often matched with black teachers. Ultimately, there seems to be no escaping the fact that schools with harsh environments will have to pay significantly higher salaries than other schools to attract their share of high-quality teachers, or will have to spend more money on professional development to upgrade the skills of the teachers they are able to attract and retain.

Improving the quality of teachers is one option for increasing the intensity of instruction offered to at-risk students; another is reducing class size. While these

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