Correcting Illegal NP Omissions Using Local Focus

1 I N T R O D U C T I O N The work described here is in the context of developing a system that will correct the written EnliSh of native users of American Sign Language SL) who are learning English as a second language. In this paper we focus on one error class that we have found to be particularly prevalent: the illegal omission of NP's. Our previous analysis of the written English of ASL natives has led us to conclude that language transfer (LT) can explain many errors, and should thus be taken advantage of by an instructional system (Suri, 1991; Suri and McCoy, 1991). We believe that many of the omission errors we have found are among the errors explainable by LT. Lillo-Martin (1991) investigates null argument structures in ASL. She identifies two classes of ASL verbs that allow different types of null argument structures. Plain verbs do not carry morphological markings for subject or object agreement and yet allow null argument structures in some contexts. These structures, she claims, are analogous to the null argument structures found in languages (like Chinese) that allow a null argument if the argument co-specifies the topic of a previous sentence (ttuang, 1984). Such languages are said to be discourseoriented languages. As it turns out, our writing samples collected from deaf writers contain many instances of omitted NP's where those NP's are the topic of a previous sentence and where the verb involved would be a plain verb in ASL. We believe these errors can be explained as a result of the ASL native carrying over conventions of (discourse-oriented) ASL to (sentence-oriented) English. If this is the case, then these omissions can be corrected if we track the topic, or, in computational linguistics terms, the local focus, and the actor focus. 2 We propose to do this by developing a modified version of Sidner's focus tracking algorithm (1979, 1983) that includes mechanisms for handling complex sentence types and illegally omitted NP's.