In a gain score study of handheld graphing calculator use by 458 high-school Algebra 1 students in two suburban school districts in Oregon and Kansas, using Key Curriculum Press’s Discovering Algebra textbook:
-The more access students had to graphing calculators, and the more instructional time in which graphing calculators were used, the higher the test scores.
-Scores were significantly higher where teachers reported receiving professional development on how to use a graphing calculator in math instruction.

Heller, J. I., D. A. Curtis, et al. (2005). The Impact of Handheld Graphing Calculator Use on Student Achievement in Algebra 1: 70