GenERRate is a tool for automatically inserting errors into text. Version 1.0 is available for downloading here.

For more information see:

Jennifer Foster and Oistein E. Andersen, 2009. GenERRate: Generating Errors for Use in Grammatical Error Detection. Proceedings of the NAACL Workshop on Innovative Use of NLP for Building Educational Applications, Boulder, Colorado. PDF

Please contact me if you have any questions, if you spot any bugs or if you have suggestions for additional features.