1. Butler , F. Eignor , D. Jones , S. McNamara , T. Suomi , B. A TOEFL 2000 framework for testing speaking. Princeton, NJ Educational Testing Service
2. Butler , F. A. Stevens , R. A validation study on second language proficiency writing descriptors for public high schools, colleges, and universities in California. Los Angeles University of California, Center for the Study of Evaluation, Graduate School of Education & Information Studies