Words unknown to the lexicon present a substantial problem to
part-of-speech tagging. In this paper we present a technique for fully
unsupervised acquisition of rules which guess possible parts of speech
for unknown words. This technique does not require specially prepared
training data, and uses instead the lexicon supplied with a tagger and
word frequencies collected from a raw corpus. Three complimentary sets
of word-guessing rules are statistically induced: prefix
morphological rules, suffix morphological rules and ending guessing
rules. The acquisition process is strongly associated with
guessing-rule evaluation methodology which is solely dedicated to the
performance of part-of-speech guessers. Using the proposed technique a
guessing-rule induction experiment was performed on the Brown Corpus
data and rule-sets, with a highly competitive performance, were
produced and compared with the state-of-the-art. To evaluate the
impact of the word-guessing component on the overall tagging
performance, it was integrated into a stochastic and a rule-based
tagger and applied to texts with unknown words.