next up previous
Next: Ngram model size Up: Experiments and Results Previous: Some simple algorithms

Tagset size

The full tagset of 37 is too large to esimate all models reliably, so we investigated using smaller tagsets. To find the optimal tagset size we tested a progression of tagset sizes starting from 37 down to 2. We used a greedy algorithm finding the best tag combination at each stage. We found that a tagset size of 23 (formed by collapsing the sub-categories of the four major categories in the original) gave the best results. The following results show the results comparing the original, the 23 size set and sets of size 3 and 2. tex2html_wrap_inline129 only distinguishes words from punctuation, and tex2html_wrap_inline131 distinguishes content words, function words and punctuation. An ngram of length 6 was used throughout (see below).

tabular69

In general our experiments showed that the optimal tagset size is between 15 and 25. Our standard tagset of 23 could be reduced slightly with a small improvement by combining rare tags (e.g. fw, foreign word) into the major categories.



Alan W Black
Tue Jul 1 17:09:00 BST 1997