JHU/APL Experiments in Tokenization and Non-Word Translation

In the past we have conducted experiments that investigate the benefits and peculiarities attendant to alternative methods for tokenization, particularly overlapping character n-grams. This year we continued this line of work and report new findings reaffirming that the judicious use of n-grams can lead to performance surpassing that of word-based tokenization. In particular we examined: the relative performance of n-grams and a popular suffix stemmer; a novel form of n-gram indexing that approximates stemming and achieves fast run-time performance; various lengths of n-grams; and the use of n-grams for robust translation of queries using an aligned parallel text. For the CLEF 2003 evaluation we submitted monolingual and bilingual runs for all languages and language pairs, multilingual runs using English as a source language, and a first attempt and cross-language spoken document retrieval. Our key findings are that shorter n-grams (n=4 and n=5) outperform a popular stemmer in non-Romance languages, that direct translation of n-grams is feasible using an aligned corpus, that translated 5-grams yield superior performance to words, stems, or 4grams, and that a combination of indexing methods is best of all.