5.6.1 How TBL Rules Are Applied

Let's look at one of the rules used by Brill's (1995) tagger. Before the rules apply, the tagger labels every word with its most-likely tag. We get these most-likely tags from a tagged corpus. For example, in the Brown corpus, race is most likely to be a noun:

$$ \begin{aligned}&P(NN|race)=.98\\&P(VB|race)=.02\\ \end{aligned} $$

This means that the two examples of race that we saw above will be coded as NN. In the first case, this is a mistake, as NN is the incorrect tag:

is/VBZ expected/VBN to/TO race/NN tomorrow/NN

In the second case this race is correctly tagged as an NN:

48) the/DT race/NN for/IN outer/JJ space/NN

After selecting the most-likely tag, Brill's tagger applies its transformation rules. As it happens, Brill's tagger learned a rule that applies exactly to this mistagging of race:

Change NN to VB when the previous tag is TO

This rule would change race/NN to race/VB in exactly the following situation, since it is preceded by to/TO:

expected/VBN to/TO race/NN $ \rightarrow $ expected/VBN to/TO race/VB