code: public double positivePredictedValue() {
      double ppv = 0;
      for(int i = 0; i < confusion.length; i++) {
        ppv += positivePredictedValue(i) * colSum(i);
      }
      return ppv / totalInstances();
    }
docs: Provides the <i>positive predicted value</i>. Aka <i>precision</i> or
  <i>specificity</i>: <code>TP / (TP+FP)</code>.
  
  @return the positive predicted value