Character confusion versus focus word-based correction of spelling and OCR variants in corpora
We present a new approach based on anagram hashing to handle globally the lexical variation in large and noisy text collections. Lexical variation addressed by spelling correction systems is primarily typographical variation. This is typically handled in a local fashion: given one particular text string some system of retrieving near-neighbors is applied, where near-neighbors are other text strings that differ from the particular string by a given number of characters. The difference in characters between the original string and one of its retrieved near-neighbors constitutes a particular character confusion. We present a global way of performing this action: for all possible particular character confusions given a particular edit distance, we sequentially identify all the pairs of text strings in the text collection that display a particular confusion. We work on large digitized corpora, which contain lexical variation due to both the OCR process and typographical or typesetting error and show that all these types of variation can be handled equally well in the framework we present. The character confusion-based prototype of Text-Induced Corpus Clean-up (ticcl) is compared to its focus word-based counterpart and evaluated on 6 years’ worth of digitized Dutch Parliamentary documents. The character confusion approach is shown to gain an order of magnitude in speed on its word-based counterpart on large corpora. Insights gained about the useful contribution of global corpus variation statistics are shown to also benefit the more traditional word-based approach to spelling correction. Final tests on a held-out set comprising the 1918 edition of the Dutch daily newspaper ‘Het Volk’ show that the system is not sensitive to domain variation.
KeywordsWord Pair Word Form Word Type Noisy Channel Lexical Variation
We are grateful to our anonymous reviewers for their rightful criticisms of our first draft. We like to thank our contacts at the kb for their support and patience: Paul Doorenbosch, Astrid Verheusen, Tineke Koster en Evelien Ket. Heartfelt thanks to scientific programmer Ko van der Sloot at ILK, whose reimplementation of our basic ideas demonstrated to us their essence. Early ticcl prototypes were developed within a Netherlands Organization for Scientific Research (NWO) Exact Sciences Hefboom project. The production version of ticcl was commissioned by the Koninklijke Bibliotheek - Den Haag. Development continues under the Stevin project SoNaR (STE07014). ticcl was turned into the online processing system ticclops with funding from CLARIN-NL (CLARIN-NL-09-011).
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