public double statistic(double[] x, double[] y) {
        final long n1 = checkSampleSize(x.length);
        final long n2 = checkSampleSize(y.length);
        final DoubleStatistics.Builder b = DoubleStatistics.builder(Statistic.MEAN, Statistic.VARIANCE);
        final DoubleStatistics s1 = b.build(x);
        final double m1 = s1.getAsDouble(Statistic.MEAN);
        final double v1 = s1.getAsDouble(Statistic.VARIANCE);
        final DoubleStatistics s2 = b.build(y);
        final double m2 = s2.getAsDouble(Statistic.MEAN);
        final double v2 = s2.getAsDouble(Statistic.VARIANCE);