Assessment materials that are cognitively appropriate for younger students are needed as more high schools introduce physics education in ninth grade. In this paper, we report findings for the first of a series of studies investigating the validity of a simplified version of the Force Concept Inventory, a widely used measure of mechanics conceptual knowledge. A Rasch model common person equating was conducted to assess the comparability of the FCI and the simplified version (SFCI). Eleventh and twelfth grade physics students responded to both the FCI and the SFCI. Results provide evidence that the two instruments are measuring the same construct, at virtually the same level of difficulty. We also report preliminary results for a current study, which examines the responses of ninth graders and upperclassmen randomly assigned the test forms. Initial results are promising, suggesting that the SFCI provides an effective accommodation for students with a need for simplified language without providing an unfair advantage to students with no need. A viable simplified version of the FCI would be an important alternative for physics educators and physics education researchers. Benefits of a simplified FCI would include improved assessment for younger students as well as English language learners.