Processing Complex Noun Phrases in a Natural Language Interface to a Statistical Database

PROCESSING COMPLEX NOUN PHRASES IN A NATURAL LANGUAGE INTERFACE TO A STATISTICAL DATABASE Fred POPOWlCH, Paul MCFETRIDGE, Dan FASS, Gary HALL School of Computing Science / Centre for Systems Science Simon Fraser University, Burnaby, B.C., Canada V5A 1S6 Abstract Analysis of a corpus of queries to a statistical database has shown considerable variation in the location and order of modifiers in complex noun phrases. Never- theless, restrictions can be defined on nominal mod- ification because of certain correspondences between nominal modifiers and the role they fulfill in a statisti- cal database, notably that the names of database tables and columns, and values of columns, are all determined by the modifiers. These restrictions are described. In- corporating these restrictions into Head-Driven Phrase Structure Grammar (HPSG) has caused us to examine the treatment of nominal modification in HPSG. A new treatment is proposed and an implementation within an HPSG based natural language front-end to a statistical database is described. 1 Introduction A prototype natural language front-end to statistical databases is being developed as part of an Execu- tive Information System for Rogers Cablesystems, a Canadian cable television company. The initial target database is the Rogers Technical Operations Database, a relational database containing statistical data describ- ing aspects of the company's business related to cus- tomer service. The front-end employs an HPSG chart parser. There axe numerous variations of HPSG; we have chosen [PS87] since it is the most familiar and widely pub- lished. Our results can be extended to other variations. In the spirit of HPSG, we have avoided a proliferation of grammar roles and kept them highly schematic. In developing the grammar for the queries in our corpus, we encountered a selection of interesting noun phrase constructions which caused us to examine the treatment of adjunct modification of nominals within HPSG. This has resulted in a proposal which should be of interest to other researchers developing natural language interfaces. 2 Complex NPs in Queries We began the project by collecting a corpus of 68 En- glish language queries from three senior executives at Rogers. Our corpus contains constructions paradig- matic of a wide selection of natural language queries that the executives would like to pose to their database. A selection of these queries are shown in (1-6). (1) Give me the we.stem region outage log summary. (2) Give me the system reliability performance. (3) Compare the basic service problem statistics per thousand customers. (4) Compare the terminal equipment problems. The sentences contain complex NP constructions and there is a large amount of variation with respect to the location and ordering of the modifiers. For example, most pre-nominal modifiers may also appear as post-nominal modifiers. (5) Vancouver system reliability performance (6) system reliability performance for Vancouver Prepositional phrases like for Vancouvercan be viewed as an abbreviated form of the prepositional phrase for the Vancouver division. The NPs within these sentences contain a great deal of syntactic ambiguity. Consider the complex NP in (1). The adjective western can either modify region or outage …

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