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

quired. We espouse the view that the initial interlanguage model formed by the learner is based largely on knowledge from his or her native language (Schwartz, 1998 Schwartz and Sprouse, 1996), and that as the learner progresses, more of the interlanguage is a correct model of the target language, and less re ects the native language or incorrect assumptions. As the interlanguage grammar progresses toward a target-like form, portions of the grammar become the focus of hypothesis testing and thus are somewhat in\ ux" between an incorrect form and a target-like form. Each\revision" of the interlanguage results in an increase in target-like features in the grammar. It is our goal to create a model whose contents will reveal the status of these features as a snapshot of the learner's current interlanguage state. Research in second language acquisition and education indicat

es that as a learner masters a subject, there is always some subset of that material that is currently within his or her grasp to acquire. Intuitively, it is this area that he or she is currently in the process of learning. This subset has been termed the Zone of Proximal Development (ZPD) (Vygotsky, 1986). The general idea has been applied to assessment and writing instruction by (Rueda, 1990), and to second language acquisition by (Krashen, 1981). In our model, the ZPD corresponds to the portion of the interlanguage currently\in ux" and in the process of making a transition to the target grammar. The identi cation of the ZPD for a given second language learner would be an ideal indication of the next language features he or she will acquire, or those features on which instruction would be most bene cial because they are neither well-established nor beyond his or her ability to learn at this time. The goal of identifying the ZPD is aided by the suggestion made by other researchers in rst and second language acquisition that the language errors committed by a learner systematically change over time (Dulay and Burt, 1974), and furthermore that there may be a speci c sequence of acquisition a learner follows in acquiring language features that may be relatively xed regardless of the native language (Krashen, 1981 Ingram, 1989 Dulay and Burt, 1975 Bailey et al., 1974). If the learner is following this sequence, identifying the current

stage of the process would have implications for which features have been learned, and which are soon to be acquired. In order to be able to determine the learner's placement in an order of acquisition of morphosyntactic features, we will examine the user's performance in the target language, and compare it against what we know of the performance of learned cognitive skills. (O'Malley and Chamot, 1990) list the stages cognitive, associative, and autonomous to explain the progression of a learner through levels of competency. We expect the\in ux" portion of our learner's interlanguage to go through similar stages. At the initial cognitive stage, a language feature has just entered the learner's ZPD. At this point, the learner is aware of the features but the knowledge of how to use them is impoverished or incomplete, so he or she is incapable of performance with consistent skill. This knowledge is thus termed\declarative." At the next stage, the associative stage, the errors in the original declarative representation are systematically deleted while the learner improves his or her understanding. The declarative knowledge develops into\procedural" form, or a form which can be used to successfully utilize the feature. At the nal autonomous stage, the performance is ne-tuned, and the skill becomes virtually automatic while errors disappear. This functional knowledge distinction is also similar to that represented in the user model of TAILOR, another tutoring system (Paris, 1987), and is supported by psychological stud

ies such as (Chi et al., 1981). The fact that declarative knowledge is shallow and results in the production of errors ties these views into the ZPD theory, where the Zone is the area in which one should expect the most errors to occur (Vygotsky, 1986). To apply these theories to second language performance, the errors produced by a learner should predominantly represent the morphosyntactic features in his or her ZPD. Features that have been acquired previously should occur without significant variation or error, and features beyond the ZPD should be absent from his or her language production because they are beyond the learner's knowledge. We will represent these observations and theories in an overlay model which is an instan-