code: public double getRMSError() {
          double r = covarCalc.getSumSquaredDeviations()
                  / Math.sqrt(varCalcX.getSumSquaredDeviations()
                  * varCalcY.getSumSquaredDeviations());
          return Math.sqrt(1 - r) * varCalcX.getStdDev();
      }
docs: Returns the r.m.s. error of this regression line.
  
  @return the r.m.s. error of this regression line.