This paper evaluates frequency and detection performance for both spelling and grammatical errors in a corpus of published Danish newspaper texts, comparing the results of three human proofreaders with those of an automatic system, DanProof. Adopting the error categorization scheme of the latter, we look at the accuracy of individual error types and their relative distribution over time, as well as the adequacy of suggested corrections. Finally, we discuss so-called artefact errors introduced by corpus processing, and the potential of DanProof as a corpus cleaning tool for identifying and correcting format conversion, OCR or other compilation errors. In the evaluation, with balanced F1-scores of 77.6 and 67.6 for 1999 texts and 2019 texts, respectively, DanProof achieved a higher recall and accuracy than the individual human annotators, and contributed the largest share of errors not detected by others (16.4% for 1999 and 23.6% for 2019). However, the human annotators had a significantly higher precision. Not counting artifacts, the overall error frequency in the corpus was low ( 0.5%), and less than half in the newer texts compared to the older ones, a change that mostly concerned orthographical errors, with a correspondingly higher relative share of grammatical errors.

This paper discusses how ML based classifiers can be enhanced disproportionately by adding small amounts of qualitative linguistic knowledge. As an example we present the Danish classifier Smatgrisene, our contribution to the recent OffensEval Challenge 2020. The classifier was trained on 3000 social media posts annotated for offensiveness, supplemented by rules extracted from the reference work on Danish offensive language (Rathje 2014b). Smatgrisene did surprisingly well in the competition in spite of its extremely simple design, showing an interesting trade-off between technological muscle and linguistic intelligence. Finally, we comment on the perspectives in combining qualitative and quantitative methods for NLP.