1. 2012b ACTFL OPIc familiarization manual https://www.languagetesting.com/wp-content/uploads/2012/07/OPIc-Familiarization-Manual.pdf
2. 2015 NCSSFL-ACTFL can-do statements http://www.actfl.org/global_statements
3. 2014 Standards for educational and psychological testing
4. A rating formulation for ordered response categories;Andrich;Psychometrika,1978