This article describes how "curriculum-based measurement--computation" (CBM-C) mathematics probes can be used in combination with "error pattern analysis" (EPA) to pinpoint difficulties in basic computation skills for students who struggle with learning mathematics. Both assessment procedures provide ongoing assessment data affording teachers up-to-date information for modification of instructional methodology and individualization of instruction based specifically on students' computation difficulties. Results from three action research projects demonstrate positive gains, with only 8 weeks of instruction, using CBM-C combined with EPA for middle school students who struggle with learning mathematics.