diff --git a/examples/preferential-optimization/generator.py b/examples/preferential-optimization/generator.py
index 1a498fe7..d8f7d3bd 100644
--- a/examples/preferential-optimization/generator.py
+++ b/examples/preferential-optimization/generator.py
@@ -23,6 +23,7 @@ os.makedirs(artifact_path, exist_ok=True)
 
 def main() -> NoReturn:
     study = create_study(
+        n_generate=5,
         study_name="Preferential Optimization",
         storage=STORAGE_URL,
         sampler=PreferentialGPSampler(),
@@ -63,9 +64,6 @@ def main() -> NoReturn:
             )
             save_note(trial, note)
 
-            # 5. Mark comparison ready
-            study.mark_comparison_ready(trial)
-
 
 if __name__ == "__main__":
     main()
diff --git a/optuna_dashboard/preferential/_study.py b/optuna_dashboard/preferential/_study.py
index 8895d604..e412d0ee 100644
--- a/optuna_dashboard/preferential/_study.py
+++ b/optuna_dashboard/preferential/_study.py
@@ -21,7 +21,6 @@ from optuna_dashboard.preferential._system_attrs import set_n_generate
 
 _logger = logging.get_logger(__name__)
 _SYSTEM_ATTR_PREFERENTIAL_STUDY = "preference:is_preferential"
-_SYSTEM_ATTR_COMPARISON_READY = "preference:comparison_ready"
 
 
 class PreferentialStudy:
@@ -230,44 +229,6 @@ class PreferentialStudy:
         """
         self._study.set_user_attr(key, value)
 
-    def mark_comparison_ready(self, trial_or_number: optuna.Trial | int) -> None:
-        """Mark trials ready to compare.
-
-        Args:
-            trial_or_number:
-                A Trial object or trial_number.
-        """
-        storage = self._study._storage
-        if isinstance(trial_or_number, optuna.Trial):
-            trial_id = trial_or_number._trial_id
-        elif isinstance(trial_or_number, int):
-            trial_id = storage.get_trial_id_from_study_id_trial_number(
-                self._study._study_id, trial_or_number
-            )
-        else:
-            raise RuntimeError("Unexpected trial type")
-        storage.set_trial_system_attr(trial_id, _SYSTEM_ATTR_COMPARISON_READY, True)
-
-    @property
-    def n_generate(self) -> int:
-        """Return the number of trials that should be generated and shown to user.
-
-        :func:`~optuna_dashboard.preferential.PreferentialStudy.should_generate` returns
-        :obj:`True` if the number of trials not reported bad and not skipped are less than
-        :attr:`~optuna_dashboard.preferential.PreferentialStudy.n_generate`.
-        """
-        system_attrs = self._study._storage.get_study_system_attrs(self._study._study_id)
-        return get_n_generate(system_attrs)
-
-    def set_n_generate(self, n_generate: int) -> None:
-        """Set the number of trials that should be generated and shown to user.
-
-        :func:`~optuna_dashboard.preferential.PreferentialStudy.should_generate` returns
-        :obj:`True` if the number of trials not reported bad and not skipped are less than
-        :attr:`~optuna_dashboard.preferential.PreferentialStudy.n_generate`.
-        """
-        return set_n_generate(self._study._study_id, self._study._storage, n_generate)
-
     def should_generate(self) -> bool:
         """Return whether the generator should generate a new trial now.
 
@@ -276,25 +237,7 @@ class PreferentialStudy:
         to generate a new trial if this method returns :obj:`True`, and to wait for human
         evaluation if this method returns :obj:`False`.
         """
-        return (
-            len(get_active_trials(self._study._study_id, self._study._storage)) < self.n_generate
-        )
-
-
-def get_active_trials(study_id: int, storage: optuna.storages.BaseStorage) -> list[FrozenTrial]:
-    preferences = get_preferences(study_id, storage)
-    worse_numbers = {worse for _, worse in preferences}
-    study_system_attrs = storage.get_study_system_attrs(study_id)
-    active_trials = []
-    for t in storage.get_all_trials(
-        study_id, deepcopy=False, states=(TrialState.COMPLETE, TrialState.RUNNING)
-    ):
-        if t.number in worse_numbers:
-            continue
-        if is_skipped_trial(t._trial_id, study_system_attrs):
-            continue
-        active_trials.append(copy.deepcopy(t))
-    return active_trials
+        return len(self.best_trials) < get_n_generate(self._study.system_attrs)
 
 
 def get_best_trials(study_id: int, storage: optuna.storages.BaseStorage) -> list[FrozenTrial]:
@@ -318,6 +261,7 @@ def get_best_trials(study_id: int, storage: optuna.storages.BaseStorage) -> list
 
 def create_study(
     *,
+    n_generate: int,
     storage: str | optuna.storages.BaseStorage | None = None,
     sampler: BaseSampler | None = None,
     study_name: str | None = None,
@@ -337,6 +281,12 @@ def create_study(
             trial = study.ask()
 
     Args:
+        n_generate:
+            The number of active trials to keep.
+            :func:`~optuna_dashboard.preferential.PreferentialStudy.should_generate` returns
+            :obj:`True` if the number of trials not reported bad and not skipped are less than
+            ``n_generate``.
+
         storage:
             Database URL. If this argument is set to None, in-memory storage is used, and the
             :class:`~optuna_dashboard.preferential.PreferentialStudy` will not be persistent.
@@ -372,7 +322,7 @@ def create_study(
         study._storage.set_study_system_attr(
             study._study_id, _SYSTEM_ATTR_PREFERENTIAL_STUDY, True
         )
-        set_n_generate(study._study_id, study._storage, 4)  # Default n_generate
+        set_n_generate(study._study_id, study._storage, n_generate)
         return PreferentialStudy(study)
 
     except optuna.exceptions.DuplicatedStudyError:
diff --git a/python_tests/preferential/test_study.py b/python_tests/preferential/test_study.py
index 3e0b011c..abc1bc4d 100644
--- a/python_tests/preferential/test_study.py
+++ b/python_tests/preferential/test_study.py
@@ -25,7 +25,7 @@ from ..storage_supplier import StorageSupplier
 @parametrize_storages
 def test_study_set_and_get_user_attrs(storage_supplier: Callable[[], StorageSupplier]) -> None:
     with storage_supplier() as storage:
-        study = create_study(storage=storage)
+        study = create_study(n_generate=4, storage=storage)
 
         study.set_user_attr("dataset", "MNIST")
         assert study.user_attrs["dataset"] == "MNIST"
@@ -34,13 +34,12 @@ def test_study_set_and_get_user_attrs(storage_supplier: Callable[[], StorageSupp
 @parametrize_storages
 def test_report_and_get_preferences(storage_supplier: Callable[[], StorageSupplier]) -> None:
     with storage_supplier() as storage:
-        study = create_study(storage=storage)
+        study = create_study(n_generate=4, storage=storage)
         assert len(study.preferences) == 0
 
         for _ in range(2):
             trial = study.ask()
             trial.suggest_float("x", 0, 1)
-            study.mark_comparison_ready(trial)
         better, worse = study.trials
         study.report_preference(better, worse)
         assert len(study.preferences) == 1
@@ -51,7 +50,9 @@ def test_report_and_get_preferences(storage_supplier: Callable[[], StorageSuppli
 
 
 def test_study_pickle() -> None:
-    study_1 = create_study()
+    study_1 = create_study(
+        n_generate=4,
+    )
     for _ in range(10):
         study_1.ask()
     assert len(study_1.trials) == 10
@@ -69,13 +70,17 @@ def test_study_pickle() -> None:
 def test_create_study(storage_supplier: Callable[[], StorageSupplier]) -> None:
     with storage_supplier() as storage:
         # Test creating a new study.
-        study = create_study(storage=storage, load_if_exists=False)
+        study = create_study(n_generate=4, storage=storage, load_if_exists=False)
 
         # Test `load_if_exists=True` with existing study.
-        create_study(study_name=study.study_name, storage=storage, load_if_exists=True)
+        create_study(
+            n_generate=4, study_name=study.study_name, storage=storage, load_if_exists=True
+        )
 
         with pytest.raises(DuplicatedStudyError):
-            create_study(study_name=study.study_name, storage=storage, load_if_exists=False)
+            create_study(
+                n_generate=4, study_name=study.study_name, storage=storage, load_if_exists=False
+            )
 
 
 @parametrize_storages
@@ -92,7 +97,7 @@ def test_load_study(storage_supplier: Callable[[], StorageSupplier]) -> None:
             load_study(study_name=study_name, storage=storage)
 
         # Create a new study.
-        created_study = create_study(study_name=study_name, storage=storage)
+        created_study = create_study(n_generate=4, study_name=study_name, storage=storage)
 
         # Test loading an existing study.
         loaded_study = load_study(study_name=study_name, storage=storage)
@@ -108,7 +113,7 @@ def test_load_study_study_name_none(storage_supplier: Callable[[], StorageSuppli
 
         study_name = str(uuid.uuid4())
 
-        _ = create_study(study_name=study_name, storage=storage)
+        _ = create_study(n_generate=4, study_name=study_name, storage=storage)
 
         loaded_study = load_study(study_name=None, storage=storage)
 
@@ -116,7 +121,7 @@ def test_load_study_study_name_none(storage_supplier: Callable[[], StorageSuppli
 
         study_name = str(uuid.uuid4())
 
-        _ = create_study(study_name=study_name, storage=storage)
+        _ = create_study(n_generate=4, study_name=study_name, storage=storage)
 
         # Ambiguous study.
         with pytest.raises(ValueError):
@@ -131,7 +136,7 @@ def test_delete_study(storage_supplier: Callable[[], StorageSupplier]) -> None:
             delete_study(study_name="invalid-study-name", storage=storage)
 
         # Test deleting an existing study.
-        study = create_study(storage=storage, load_if_exists=False)
+        study = create_study(n_generate=4, storage=storage, load_if_exists=False)
         delete_study(study_name=study.study_name, storage=storage)
 
         # Test failed to delete the study which is already deleted.
@@ -141,12 +146,11 @@ def test_delete_study(storage_supplier: Callable[[], StorageSupplier]) -> None:
 
 def test_copy_study() -> None:
     with StorageSupplier("sqlite") as from_storage, StorageSupplier("sqlite") as to_storage:
-        from_study = create_study(storage=from_storage)
+        from_study = create_study(n_generate=4, storage=from_storage)
         from_study.set_user_attr("baz", "qux")
         for _ in range(3):
             trial = from_study.ask()
             trial.suggest_float("x", 0, 1)
-            from_study.mark_comparison_ready(trial)
         from_study.report_preference(from_study.trials[0], from_study.trials[1])
         from_study.report_preference(from_study.trials[1], from_study.trials[2])
 
@@ -165,8 +169,8 @@ def test_copy_study() -> None:
 
 def test_copy_study_to_study_name() -> None:
     with StorageSupplier("sqlite") as from_storage, StorageSupplier("sqlite") as to_storage:
-        from_study = create_study(study_name="foo", storage=from_storage)
-        _ = create_study(study_name="foo", storage=to_storage)
+        from_study = create_study(n_generate=4, study_name="foo", storage=from_storage)
+        _ = create_study(n_generate=4, study_name="foo", storage=to_storage)
 
         with pytest.raises(DuplicatedStudyError):
             copy_study(
@@ -188,7 +192,7 @@ def test_copy_study_to_study_name() -> None:
 @parametrize_storages
 def test_add_trial(storage_supplier: Callable[[], StorageSupplier]) -> None:
     with storage_supplier() as storage:
-        study = create_study(storage=storage)
+        study = create_study(n_generate=4, storage=storage)
         assert len(study.trials) == 0
 
         trial = create_trial(value=0)
@@ -198,7 +202,9 @@ def test_add_trial(storage_supplier: Callable[[], StorageSupplier]) -> None:
 
 
 def test_add_trial_invalid_values_length() -> None:
-    study = create_study()
+    study = create_study(
+        n_generate=4,
+    )
     trial = create_trial(values=[0, 0])
     with pytest.raises(ValueError):
         study.add_trial(trial)
@@ -207,7 +213,7 @@ def test_add_trial_invalid_values_length() -> None:
 @parametrize_storages
 def test_add_trials(storage_supplier: Callable[[], StorageSupplier]) -> None:
     with storage_supplier() as storage:
-        study = create_study(storage=storage)
+        study = create_study(n_generate=4, storage=storage)
         assert len(study.trials) == 0
 
         study.add_trials([])
@@ -220,7 +226,7 @@ def test_add_trials(storage_supplier: Callable[[], StorageSupplier]) -> None:
             assert trial.number == i
             assert trial.value == i
 
-        other_study = create_study(storage=storage)
+        other_study = create_study(n_generate=4, storage=storage)
         other_study.add_trials(study.trials)
         assert len(other_study.trials) == 3
         for i, trial in enumerate(other_study.trials):
@@ -231,11 +237,10 @@ def test_add_trials(storage_supplier: Callable[[], StorageSupplier]) -> None:
 @parametrize_storages
 def test_get_trials(storage_supplier: Callable[[], StorageSupplier]) -> None:
     with storage_supplier() as storage:
-        study = create_study(storage=storage)
+        study = create_study(n_generate=4, storage=storage)
         for _ in range(5):
             trial = study.ask()
             trial.suggest_int("x", 1, 5)
-            study.mark_comparison_ready(trial)
 
         with patch("copy.deepcopy", wraps=copy.deepcopy) as mock_object:
             trials0 = study.get_trials(deepcopy=False)
@@ -256,10 +261,9 @@ def test_get_trials(storage_supplier: Callable[[], StorageSupplier]) -> None:
 @parametrize_storages
 def test_get_trials_state_option(storage_supplier: Callable[[], StorageSupplier]) -> None:
     with storage_supplier() as storage:
-        study = create_study(storage=storage)
+        study = create_study(n_generate=4, storage=storage)
         for _ in range(3):
             trial = study.ask()
-            study.mark_comparison_ready(trial)
         better, worse = study.trials[:2]
         study.report_preference(better, worse)
 
@@ -286,7 +290,9 @@ def test_get_trials_state_option(storage_supplier: Callable[[], StorageSupplier]
 
 
 def test_ask() -> None:
-    study = create_study()
+    study = create_study(
+        n_generate=4,
+    )
 
     trial = study.ask()
     assert isinstance(trial, Trial)
@@ -298,7 +304,9 @@ def test_ask_fixed_search_space() -> None:
         "y": distributions.CategoricalDistribution(["bacon", "spam"]),
     }
 
-    study = create_study()
+    study = create_study(
+        n_generate=4,
+    )
     trial = study.ask(fixed_distributions=fixed_distributions)
 
     params = trial.params
@@ -312,7 +320,7 @@ def test_report_preferences_from_another_process() -> None:
 
     with StorageSupplier("sqlite") as storage:
         # Create a study and ask for a new trial.
-        study = create_study(storage=storage)
+        study = create_study(n_generate=4, storage=storage)
         study.ask()
         study.ask()
 
diff --git a/python_tests/test_api.py b/python_tests/test_api.py
index fc67e12e..ba3b0dc4 100644
--- a/python_tests/test_api.py
+++ b/python_tests/test_api.py
@@ -102,10 +102,9 @@ class APITestCase(TestCase):
 
     def test_get_best_trials_of_preferential_study(self) -> None:
         storage = optuna.storages.InMemoryStorage()
-        study = create_study(storage=storage)
+        study = create_study(n_generate=4, storage=storage)
         for _ in range(3):
             trial = study.ask()
-            study.mark_comparison_ready(trial)
         study.report_preference(study.trials[0], study.trials[1])
 
         assert len(study.best_trials) == 1
@@ -126,10 +125,9 @@ class APITestCase(TestCase):
 
     def test_report_preference(self) -> None:
         storage = optuna.storages.InMemoryStorage()
-        study = create_study(storage=storage)
+        study = create_study(n_generate=4, storage=storage)
         for _ in range(3):
             trial = study.ask()
-            study.mark_comparison_ready(trial)
 
         app = create_app(storage)
         study_id = study._study._study_id
@@ -154,11 +152,10 @@ class APITestCase(TestCase):
 
     def test_skip_trial(self) -> None:
         storage = optuna.storages.InMemoryStorage()
-        study = create_study(storage=storage)
+        study = create_study(n_generate=4, storage=storage)
         trials: list[optuna.Trial] = []
         for _ in range(3):
             trial = study.ask()
-            study.mark_comparison_ready(trial)
             trials.append(trial)
 
         app = create_app(storage)
diff --git a/python_tests/test_serializers.py b/python_tests/test_serializers.py
index a75db32d..a90e0de7 100644
--- a/python_tests/test_serializers.py
+++ b/python_tests/test_serializers.py
@@ -24,7 +24,7 @@ def test_serialize_dict() -> None:
 
 def test_get_study_detail_is_preferential() -> None:
     storage = optuna.storages.InMemoryStorage()
-    study = create_study(storage=storage)
+    study = create_study(n_generate=4, storage=storage)
     study_summaries = get_study_summaries(storage)
     assert len(study_summaries) == 1
 
@@ -46,7 +46,7 @@ def test_get_study_detail_is_not_preferential() -> None:
 
 def test_get_study_summary_is_preferential() -> None:
     storage = optuna.storages.InMemoryStorage()
-    create_study(storage=storage)
+    create_study(n_generate=4, storage=storage)
     study_summaries = get_study_summaries(storage)
     assert len(study_summaries) == 1