Improving Part of Speech Disambiguation Rules by Adding Linguistic Knowledge

This paper reports the ongoing work of producing a state of the art part of speech tagger for unedited Swedish text. Rules eliminating faulty tags have been induced using Progol. In previously reported experiments, almost no linguistically motivated background knowledge was used [5,8]. Still, the result was rather promising (recall 97.7%, with a pending average ambiguity of 1.13 tags/word). Compared to the previous study, a much richer, more linguistically motivated, background knowledge has been supplied, consisting of examples of noun phrases, verb chains, auxiliary verbs, and sets of part of speech categories. The aim has been to create the background knowledge rapidly, without laborious hand-coding of linguistic knowledge. In addition to the new background knowledge, new, more expressive rule types have been induced for two part of speech categories and compared to the corresponding rules of the previous bottom-line experiment. The new rules perform considerably better, with a recall of 99.4% for the new rules, compared to 97.6% for the old rules. Precision was slightly better for the new rules.