Abstract: We present recent developments of Korektor, a statistical spell checking system. In addition to lexicon, Korektor uses language models to find real-word errors, detectable only in context. The models and error probabilities, learned from error corpora, are also used to suggest the most likely corrections. Korektor was originally trained on a small error corpus and used language models extracted from an in-house corpus WebColl. We show two recent improvements: • We built new language models from freely available (shuffled) versions of the Czech National Corpus and show that these perform consistently better on texts produced both by native speakers and nonnative learners of Czech. • We trained new error models on a manually annotated learner corpus and show that they perform better than the standard error model (in error detection) not only for the learners' texts, but also for our standard evaluation data of native Czech. For error correction, the standard error model outperformed non-native models in 2 out of 3 test datasets. We discuss reasons for this not-quite-intuitive improvement. Based on these findings and on an analysis of errors in both native and learners' Czech, we propose directions for further improvements of Korektor
