docs: @Reference(authors = "W. M. Rand", //
        title = "Objective Criteria for the Evaluation of Clustering Methods", //
        booktitle = "Journal of the American Statistical Association, Vol. 66 Issue 336", //
        url = "https://doi.org/10.2307/2284239", //
        bibkey = "doi:10.2307/2284239")
    public double randIndex() {
      final double sum = pairconfuse[0] + pairconfuse[1] + pairconfuse[2] + pairconfuse[3];
      return (pairconfuse[0] + pairconfuse[3]) / sum;
    }