Another issue, related to vertical domain restriction, is standardization and comparability between systems. Most of the ITSs reported in literature do not use some generalized framework of ability reference (such as CEFR or ILR scale) to define the level of learner proficiency for which the learning content of the ITS is intended, but specify such a level in terms of school grade or educational level only (e.g., college freshmen course). This significantly affects learner mobility between different ITSs, mostly because learners are unable to
objectively assess how appropriate the language learning system is for their proficiency level. Furthermore, the results of using an ITS for language learning should be observable as a shift in the proficiency level, quantified in familiar terms, and available for analysis for third parties. Aligning the content with any of the internationally set and verified standards for language learning proficiency levels can be a way to successfully address such issues.
output.