@Test
public void testReturnNaNDoubleWhenNoSimilaritiesForTwoItems() throws Exception {

 DataModel dataModel = getDataModel(
          new long[] {1, 2},
          new Double[][] {
                  {null, null, 3.0},
                  {1.0, 1.0, null},
          });  
 Double similarity = new TanimotoCoefficientSimilarity(dataModel).itemSimilarity(1, 2);
 assertEquals(Double.NaN, similarity, EPSILON);
}