This article presents a method for pre-processing the feature vectors representing text documents that are consequently classified using unsupervised methods. The main goal is to show that state-of-the-art classification methods can be improved by a certain data preparation process. The first method is a standard K-means clustering and the second Latent Dirichlet allocation (LDA) method. Both are widely used in text processing. The mentioned algorithms are applied to two data sets in two different languages. First of them, the 20NewsGroup is a widely used benchmark for classification of English documents. The second set was selected from the large body of Czech news articles and was used mainly to compare the performance of the tested methods also for the case of less frequently studied language. Furthermore, the unsupervised methods are also compared with the supervised ones in order to (in some sense) ascertain the upper-bound of the task.
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