code: public double mcc() {
      if(!isBinary())throw new UnsupportedOperationException("precision is only implemented for 2 class problems.");
      double tn = _arr[0][0];
      double fp = _arr[0][1];
      double tp = _arr[1][1];
      double fn = _arr[1][0];
      double mcc = (tp*tn - fp*fn)/Math.sqrt((tp+fp)*(tp+fn)*(tn+fp)*(tn+fn));
      return mcc;
    }
docs: The Matthews Correlation Coefficient, takes true negatives into account in contrast to F-Score
  See <a href="http://en.wikipedia.org/wiki/Matthews_correlation_coefficient">MCC</a>
  MCC = Correlation between observed and predicted binary classification
  @return mcc ranges from -1 (total disagreement) ... 0 (no better than random) ... 1 (perfect)