The Lexical Semantics of Comparative Expressions in a Multi-Level Semantic Processor

Comparative expressions (CEs) such as "bigger than" and "more oranges than" are highly ambiguous, and their meaning is context dependent. Thus, they pose problems for the semantic interpretation algorithms typically used in natural language database interfaces. We focus on the comparison attribute ambiguities that occur with CEs. To resolve these ambiguities our natural language interface interacts with the user, finding out which of the possible interpretations was intended. Our multi-level semantic processor facilitates this interaction by recognizing the occurrence of comparison attribute ambiguity and then calculating and presenting a list of candidate comparison attributes from which the user may choc6e. I I PROBLEM DESCRIPTION. Although there has been considerable work on the development of natural language database interfaces, many difficult language interpretation problems remain. One of these is the semantic interpretation of comparative expressions such as those shown in sentences (1) through (3). (1) Does ACME construct b e t t e r buildings than ACE? (2) Does ACME construct buildings faster than ACE? (3) Are more oranges than apples exported by Mexico? To interpret a comparative expression (CE) a 'natural language processor must determine (1) the entities to he compared, and (2) the attribute(s) of those entities to consider in performing the comparison. The selection of comparison attributes is made difficult by the high level of lexical ambiguity exhibited by comparative predicates. For example, what pieces of data should be compared to answer query (1)? If the database contains information about foundation type, structural characteristics, wiring, and insulation, any of these attributes could be used. Similarly, when comparing orange and apple exports as in query (3), we might compare numeric quantity, weight, volume, or monetary value. To further complicate matters, the plausible comparison attributes for a comparative predicate change with the arguments to which that predicate is applied. Table 1 shows several examples of likely comparison attributes to use with the predicate "bigger" depending on the types of entity that are being compared. Since the system must determine for a comparative predicate the lexical definition intended by the user, this problem is, at heart, one of lexical ambiguity resolution. The problems discussed so far are similar to the well known vagueness and context sensitivity of adjectives (although they occur here even in sentences without adjectives such as (3)). Any proposed method of CE interpretation should also treat several other phenomena that are unique to comparatives. These are bipredicational comparisons, cross-class comparisons, and pairability constraints. Bipredlca t ional comparisons involve two predicates, as shown in example (4) (the