According to the study done by Smarter Balanced Assessment Consortium, using word to word dictionary as Test accommodations for ELLs shows testing improvement of 0.247 mean. (24.7% improvement)*. Most states allow their ELL/ LEP students to use a word to word dictionary as an accommodation or variations during their state standardized tests (CST, STAAR, Regents, FCAT, AIMS etc). The policy regarding the use of a word to word dictionary during testing differs from state to state. While some states have general guidelines for using word to word dictionaries during testing, others have very specific requirements.
* (Smarter Balanced Assessment Consortium: Summary of Literature on Empirical Studies of the Validity and Effectiveness of Test Accommodations for ELLs: 2005–2012 http://www.smarterbalanced.org/wordpress/wp-content/uploads/2012/08/Smar...)