// assumes value1 and value2 are Missing attributes and not equal
  private String returnHigherPrecedenceMissingAttribute(String value1, String value2) {
    if (value1.equals(MissingAttributeValues.NULL.toString())
        && MissingAttributeValues.has(value2)
        && !value2.equals(MissingAttributeValues.NULL.toString())) {
      return value1;
    } else if (value2.equals(MissingAttributeValues.NULL.toString())
        && MissingAttributeValues.has(value1)
        && !value1.equals(MissingAttributeValues.NULL.toString())) {
      return value2;
    }

    assert true == false
        : String.format("returnHigherPrecendenceMissingAttribute, %s, %s", value1, value2);
    return MissingAttributeValues.MISSING.toString();
  }
 private boolean patientIsDiseaseFree(DiseaseFreeData df) {
   return (VitalStatusAlive.has(df.vitalStatus)
       && (df.daysToNewTumorEventAfterInitialTreatment.equals(
               MissingAttributeValues.NULL.toString())
           || df.daysToNewTumorEventAfterInitialTreatment.equals(
               MissingAttributeValues.getNotAvailable())));
 }