The pressure to meet the demands of "No Child Left Behind" (NCLB) Act coupled with poor results by secondary students on national assessments in mathematics have forced school principals to develop skill sets in the use of data in efforts to increase student performance on standardized assessments. The effective use of data by school principals has been shown to improve student performance. This study compared the use of data systems by secondary principals, and their perceptions of data type and data tool effectiveness in enhancing student achievement in mathematics in high and low performing schools. One-hundred forty-seven secondary school principals in high and low performing schools, as measured by the percentage of grade 11 students rated proficient on the 2008-2009 PSSA standardized mathematics assessments, were surveyed to examine what principals in high performing schools do differently in their use of data to increase student performance in mathematics as measured by standardized assessments. Analysis of the survey focused on 11 sub-categories that included: use of input data; use of process data; use of outcomes data; use of satisfaction data; perceived effectiveness of input data; perceived effectiveness of process data; perceived effectiveness of outcomes data; perceived effectiveness of satisfaction data; use of data systems; use of data tools; and, perceived effectiveness of data tools used. Significant differences were found between principals in high and low performing schools in their use of data systems and overall use of data tools. Principals in lower performing schools were found to use data systems to a greater extent and reported a significantly higher use of data tools in their efforts to improve mathematics scores. Chi-square analysis showed some significant differences between principal groups in the use of benchmark assessments and related data tools. The researcher concluded that principals in low performing schools see the use of data as the cure for poor student achievement in mathematics as they feel the pressure of high stakes accountability under NCLB. Pressure to increase student achievement may have resulted in greater use of data tools and systems as principals in low performing schools search for solutions to poor student achievement. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]