This study examined the effect of linguistic modification on middle school students' ability to show what they know and can do on math assessments. REL West's study on middle school math assessment accommodations found that simplifying the language--or linguistic modification--on standardized math test items made it easier for English Language learners to focus on and grasp math concepts, and thus was a more accurate assessment of their math skills. The results contribute to the body of knowledge informing assessment practices and accommodations appropriate for English language learner students. The study examined students' performance on two sets of math items--both the originally worded items and those that had been modified. Researchers analyzed results from three subgroups of students--English learners (EL), non-English language arts proficient (NEP), and English language arts proficient (EP) students. Key results include: (1) Linguistically modifying the language of mathematics test items did not change the math knowledge being assessed; (2) The effect of linguistic modification on students' math performance varied between the three student subgroups. The results also varied depending on how scores were calculated for each student; and (3) For each of the four scoring approaches analyzed, the effect of linguistic modification was greatest for EL students, followed by NEP and EP students. The report is structured as follows. Following an Executive Summary and a Study Overview, Chapter 2 describes the study design, sample selection and recruitment, item set development processes, and standardized administration procedures. Chapter 3 describes the implementation of the accommodation (linguistic modification), including discussion of considerations and methods for data analysis. Chapter 4 presents findings from data analyses. Chapter 5 summarizes and interprets key findings, describes study challenges, comments on implications of the findings, and offers recommendations for future research. Appendices include: (1) Power analysis for primary research questions; (2) Operational test administration manual; (3) Student Language Background Survey; (4) Guide for developing a linguistically modified assessment; (5) Workgroup training materials; (6) Overview and protocol for cognitive interviews; (7) Item parameter estimates for IRT models; (8) Descriptive statistics from four scoring approaches; (9) ANOVA findings across four scoring approaches; (10) Cross-approach comparisons; (11) Results of the classical item-level analyses; (12) Summary of differential item functioning findings; (13) Exploratory factor analysis results; (14) Operational item set--original; and (15) Operational item set--linguistically modified. (Contains 31 tables, 10 figures, and 45 footnotes.)