In this paper we discuss a proposed user knowledge modeling architecture for the ICICLE system, a language tutoring application for deaf learners of written English. The model will represent the language pro ciency of the user and is designed to be referen

line given thus far of what observations a ect the model is fairly vague: features used\consistently correctly" are mastered concepts, those used\consistently incorrectly" are at the current level, and those which are rare or absent from the student's writing are above his or her level. The judgment of whether something is at or below a student's level may change when the amount of data increases as the system goes through more than one piece of the student's work, particularly if one or more of the pieces is too short to contain several instances of the feature. Therefore, it makes sense for the model to track certain gures (the number of times a feature is attempted, and the number of times it is executed without error) across more than one piece of writing and to make distinctions between gures collected within the most recent piece of writing and those collected across others in the past (since the user's pro ciency will not change within a given piece, but there may be change across a selection of them). This will allow the system to examine as much data as possible, strengthening its ability to make these judgments. In this view, the user's writing is seen as a continuum of performance events over time from the rst session to the most recent. But since the user's pro ciency is also changing, the system should not always compute performance

statistics which include events stretching back to the beginning of his or her use of the system, when the performance levels may have been di erent. Therefore, we recommend that the system maintain a\sliding window" of performance history across writing samples from which to update the user model at each new analysis. Ideally, this window would include enough data to be robust, and yet be small enough to capture only the\current" statistics. This latter requirement is particularly important for the system's self-evaluation and deciding whether recent explanatory attempts have succeeded. Determining what size such a window should be is a realm of future research. Related issues are whether or not it should adjust its size according to the circumstance, and what statistics of successful execution would be su cient for judging a feature to be\consistent" in its use.

6 ConclusionsIn this paper, we have discussed a proposed architecture for a model of user second language pro ciency for use in a second language learning system. Our design is based on theories of second language acquisition research, particularly those involving interlanguage progression and concepts of orders of acquisition. We have also based the design on theories of cognitive skill acquisition by Vygotsky and others. The architecture we propose consists of hierarchies representing linear progressions of language feature acquisition, connected by lateral relationships of concurrent learning. Finally, we have shown that such a model, supported by preliminary empirical ndings, can provide a rich and robust information base to a language assessment/ correction application by modeling user pro ciency at a high level of granularity and speci city.

References

N. Bailey, C. Madden, and S. D. Krashen. 1974. Is there a`natural sequence' in adult second language learning? Language Learning, 24(2):235{243. C. Baker and D. Cokely. 1980. American Sign Language: A Teacher's Resource Text on Grammar and Culture. TJ Publishers, Silver Spring, MD. Giuseppe Carenini and Johanna D. Moore. 1993. Generating explanations in context. In Wayne D. Gray, William E. He ey, and Dianne Murray, editors, Proceedings of the International Workshop on Intelligent User Interfaces, pages 175{182, Orlando, Florida, January 4-7. ACM Press. Michelene T. H. Chi, Robert Glaser, and Ernest Rees. 1981. Expertise in problem solving. In Advances in the Psychology of Human Intelligence, chapter 1, pages 7{76. Lawrence Erlbaum, Hillsdale, NJ. Vivian Cook. 1993. Linguistics and Second Language Acquisition. Macmillan Press Ltd, London. Heidi C. Dulay and Marina K. Burt. 1974. Errors and strategies in child second language acquisition. TESOL Quarterly, 8(2):129{136, June.