Disambiguation of morphological analysis in Bantu languages

The paper describes problems in disambiguating the morphological analysis of Bantu languages by using Swahili as a test language. The main factors of ambiguity in this language group can be traced to the noun class structure on one hand and to the bi-directional word-formation on the other. In analyzing word-forms, the system applied utilizes SWATWOL, a morphological parsing program based on two-level formalism. Disambiguation is carried out with the latest version (April 1996) of the Constraint Grammar Parser (GGP). Statistics on ambiguity are provided. Solutions tbr resolving different types of ambiguity are presented and they are demonstrated by examples fi'om corpus text. Finally, statistics on the performance of the disambiguator are presented. I n t r o d u c t i o n There are five principal factors in Bantu languages which contribute to ambiguous analysis of wordtbrms. First, nouns are grouped into more than ten marked noun classes. The marking of these classes extends across the noun phrase, whereby the noun governs the choice of markers in dependent constituents. Second, verbs inflect steminitially and mark the subject, object, and relative referent by prefixes, whereby the actual form of each prefix is governed by the noun class of the noun it refers to. In addition, verb derivation also adds to the complexity of verbal morphology. Third, reduplication is a productive phenomenon. Because its accurate description in lexicon is not possible, alternative ways in handling it are discussed. Fourth, the majori ty of Bantu languages have a tone system, but rarely this is indicated in writing. This adds to morphological ambiguity. Fifth, various semantic functions of word-forms are also a source of ambiguity. In this paper I shall discuss the points one and two by using Swahili as a test language. 1 M o r p h o l o g i c a l analysis The morphological analysis of Swahili is carried out by SWATWOL, which is based on the twolevel formalism (Koskenniemi 1983). The application of this formalism to Swahili has been under process since 1987, and it has now, after having been tested with a corpus of one million words, reached a mature phase with a recall of 99.8% in average running text, and precision of close to 100%. The performance of SWATWOL corresponds to what is reported of ENGTWOL, the morphological parser of English (Voutilainen et al 1992; Tapanainen and J/irvinen 1994), and SWETWOL, the morphological analyzer of Swedish (Karlsson 1992). SWATWOL uses a two-level rule system for describing morphophonological variation, as well as a lexicon with 288 sub-lexicons. Unlike in languages with right-branching word formation, where word roots can be grouped together into a root lexicon, here word roots have been divided into several sub-lexicons. Because SWATWOL has been described in detail elsewhere (Hurskainen 1992), only a sketchy description of its parts is given here. 1.1 S W A T W O L rules Two-level rules have been written mainly for handling morhophonological processes, which occur principally in morpheme boundaries. Part of such processes take place also in verbal extensions, whereby the quality of the stem vowel(s) defines the surface form of the suffix. The total number of rules is 18, part of them being combined rules. An example of a combined rule: