Measuring Lexical Quality of a Historical Finnish Newspaper Collection ― Analysis of Garbled OCR Data with Basic Language Technology Tools and Means

Measuring Lexical Quality of a Historical Finnish Newspaper Collection – Analysis of Garbled OCR Data with Basic Language Technology Tools and Means Kimmo Kettunen, Tuula Pääkkönen National Library of Finland, The Centre for Preservation and Digitisation Saimaankatu 6, FI-50100 Mikkeli, Finland kimmo.kettunen@helsinki.fi, tuula.paakkonen@helsinki.fi Abstract The National Library of Finland has digitized a large proportion of the historical newspapers published in Finland between 1771 and 1910 (Bremer-Laamanen 2001). This collection contains approximately 1.95 million pages in Finnish and Swedish. Finnish part of the collection consists of about 2.39 billion words. The National Library’s Digital Collections are offered via the digi.kansalliskirjasto.fi web service, also known as Digi. Part of this material is also available freely downloadable in The Language Bank of Finland provided by the Fin-CLARIN consortium. The collection can also be accessed through the Korp environment that has been developed by Språkbanken at the University of Gothenburg and extended by FIN-CLARIN team at the University of Helsinki to provide concordances of text resources. A Cranfield-style information retrieval test collection has been produced out of a small part of the Digi newspaper material at the University of Tampere (Järvelin et al., 2015). The quality of the OCRed collections is an important topic in digital humanities, as it affects general usability and searchability of collections. There is no single available method to assess the quality of large collections, but different methods can be used to approximate the quality. This paper discusses different corpus analysis style ways to approximate the overall lexical quality of the Finnish part of the Digi collection. Keywords: OCR evaluation, historical newspaper collections, Finnish 1. Introduction Digitization of both hand-written and printed historical text material during the last 10–15 years has been an ongoing academic and non-academic industry. Most probably this activity will only increase in the current Digital Humanities era. As a result of the past and current work we have lots of digital historical document collections available and will have more of them in the future. This paper discusses different corpus analysis style ways to approximate the overall lexical quality of the Finnish part of the Digi collection. Methods include usage of parallel samples and word error rates, usage of morphological analysers, frequency analysis of words and comparisons to comparable lexical data. Our aim in the quality analysis is to establish a set of available simple assessment methods that also build up a compact procedure for quality assessment after e.g. re-OCRing or post-correction of the material. In the conclusion part of the paper we shall synthesise results of our different analyses. 2. Problems of Optical Character Recognition Newspapers of the 19th and early 20th century were mostly printed in the Gothic (Fraktur, blackletter) typeface in Europe. It is well known that the typeface is difficult to recognize for OCR software (Holley, 2008; Furrer & Volk, 2011; Volk et al., 2011). Other aspects that affect the quality of the OCR recognition are the following, among others (cf. Holley, 2008; Klijn, 2008, for a more detailed  list): quality of …

Content truncated to 500 words