PhD thesis - School of Informatics - University of Edinburgh

Chapter 3. Tracking English Inclusions in German 89
TreeTagger’s performance when trained on either the NEGRA or the TIGER corpus.
As reported in the system description in Section 3.3, the final full English inclu-
sion classifier incorporates TnTTIGER as the POS tagger in the pre-processing step and
Yahoo in the search engine module. The decision to use TnTTIGER was made due to
the fact that this module results in a drastic performance increase over the TnTNEGRA
module for the space travel and EU domains of 7.91 and 6.73 points in F-score, re-
spectively. On the internet data, the TnTT IGER and the TnTNEGRA modules perform
very similarly. It can therefore be concluded that a POS tagger trained on a sufficiently
large corpus is a vital component of the English inclusion classifier. In the following
section, the decision to use Yahoo in the search engine module is motivated.
3.5.2 Task-based Evaluation of Different Search Engines
Tokens that are not found in the German or English lexical database are passed to
a back-off search engine module (Section 3.3.4). Such tokens are queried just for
German and just for English webpages, a language preference that is offered by most
search engines, and classified based on the maximum normalised score of the number
of hits returned for each language. This module therefore relies to some extent on the
search engine’s internal language identification algorithm.
During the initial stages of developing the English inclusion classifier, Google was
used in the search engine module (Alex and Grover, 2004; Alex, 2005). The main
reasons for opting for Google was that it was the biggest search engine available at
the time spanning over 8 billion webpages. It also offers the language preference set-
ting which is essential for determining counts. Moreover, queries can be automated
by means of the Google Soap Search API (beta) which allows 1,000 queries per day. 11
During the course of developing the English inclusion classifier, Yahoo, another search
engine, also made an API available which allows 5,000 searches per day. 12 In Yahoo,
searches can also be restricted to webpages of a particular language. The only dif-
ferences between the two search engines is their number of indexed webpages. In
August 2005, Yahoo announced that it indexes more than 19.2 billion documents 13
which amounts to more than double the number of webpages (8.2 billion) indexed by
11 http://www.google.com/apis/
12 http://developer.yahoo.com/
13 http://www.ysearchblog.com/archives/000172.html