EVANSTON, Ill. --- A Northwestern University study investigating the effects of class size on the achievement gap between high and low academic achievers suggests that high achievers benefit more from small classes than low achievers, especially at the kindergarten and first grade levels.

"While decreasing class size may increase achievement on average for all types of students, it does not appear to reduce the achievement gap within a class," said Spyros Konstantopoulos, assistant professor at Northwestern's School of Education and Social Policy.

Konstantopoulos' study, which appears in the March issue of Elementary School Journal, questions commonly held assumptions about class size and the academic achievement gap -- one of the most debated and perplexing issues in education today.

The Northwestern professor worked with data from Project STAR, a landmark longitudinal study launched in 1985 by the State of Tennessee to determine whether small classes positively impacted the academic achievement of students.

Considered one of the most important investigations in education, STAR made it abundantly clear that on average small classes had a positive impact on the academic performance of all students.

For most school advocates, parents and policy makers, that finding was enough to call for smaller class size. However, Konstantopoulos found that that the children who already were high achievers were the primary beneficiaries of the extra attention smaller classes afforded.

"It is likely that high achievers are more engaged in learning opportunities and take advantage of the teaching practices that take place in smaller classes, or that they create opportunities for their own learning in smaller classes," said Konstantoupoulos.

"Given that class size reduction is an intervention that benefits all students, it's tempting to expect that it also will reduce the achievement gap," he added. Previous research, however, has provided weak or no evidence that class reduction benefited lower-achieving students more than others. The Northwestern study underscores that research.

The Northwestern study findings suggest that small classes produce significantly higher variability in achievement than regular classes in kindergarten mathematics and in first grade reading. Overall the results indicate that class size reduction increases not only achievement for all students on average, but the variability in student achievement as well.

"It is unfortunate that data about classroom practices that could be useful in identifying ways of improving academic success for lower achieving students were not available in Project STAR," Konstantopoulos said. "A new randomized experiment with the objective of collecting high-quality observational data in the classrooms would provide invaluable information about the effects of small sizes."

A copy of the study can be downloaded for free at http://www.journals.uchicago.edu/toc/esj/current