@Test
public void testNoSimilarity() throws Exception {

  DataModel dataModel = getDataModel(
      new long[] {1, 2, 3, 4},
      new Double[][] {
              {1.0, null, 1.0, 1.0},
              {1.0, null, 1.0, 1.0},
              {null, 1.0, 1.0, 1.0},
              {null, 1.0, 1.0, 1.0},
      });
  LogLikelihoodSimilarity similarity = new LogLikelihoodSimilarity(dataModel);
  assertCorrelationEquals(Double.NaN, similarity.itemSimilarity(1, 0));
  assertCorrelationEquals(Double.NaN, similarity.itemSimilarity(0, 1));
  assertCorrelationEquals(0.0, similarity.itemSimilarity(2, 3));
  assertCorrelationEquals(0.0, similarity.itemSimilarity(3, 2));
}