docs: @Reference(authors = "M. Steinbach, G. Karypis, V. Kumar", //
        title = "A Comparison of Document Clustering Techniques", //
        booktitle = "KDD workshop on text mining. Vol. 400. No. 1", //
        url = "http://glaros.dtc.umn.edu/gkhome/fetch/papers/docclusterKDDTMW00.pdf", //
        bibkey = "conf/kdd/SteinbachKK00")
    public double f1Measure() {
      return Util.f1Measure(purity(), inversePurity());
    }