@Test
 public void testCovariance() {
   DataFrame df = context.table("testData2");
   Double result = df.stat().cov("a", "b");
   Assert.assertTrue(Math.abs(result) < 1.0e-6);
 }
 @Test
 public void testCorrelation() {
   DataFrame df = context.table("testData2");
   Double pearsonCorr = df.stat().corr("a", "b", "pearson");
   Assert.assertTrue(Math.abs(pearsonCorr) < 1.0e-6);
 }