diff --git a/src/iterative_ensemble_smoother/experimental.py b/src/iterative_ensemble_smoother/experimental.py
index 85eab9d6..e3b772f4 100644
--- a/src/iterative_ensemble_smoother/experimental.py
+++ b/src/iterative_ensemble_smoother/experimental.py
@@ -4,7 +4,7 @@
 """
 import numbers
 import warnings
-from typing import Any, Callable, Dict, Generator, List, Optional, Tuple, Union
+from typing import Callable, List, Optional, Tuple, Union
 
 import numpy as np
 import numpy.typing as npt
@@ -17,43 +17,6 @@
 )
 
 
-def groupby_indices(
-    X: npt.NDArray[Any],
-) -> Generator[Dict[npt.NDArray[np.double], npt.NDArray[np.int_]], None, None]:
-    """Yield pairs of (unique_row, indices_of_row).
-
-    Examples
-    --------
-    >>> X = np.array([[1, 0],
-    ...               [1, 0],
-    ...               [1, 1],
-    ...               [1, 1],
-    ...               [1, 0]])
-    >>> list(groupby_indices(X))
-    [(array([1, 0]), array([0, 1, 4])), (array([1, 1]), array([2, 3]))]
-
-    Another example:
-    >>> X = np.array([[1, 2, 3],
-    ...               [0, 0, 0],
-    ...               [1, 2, 3],
-    ...               [1, 1, 1],
-    ...               [1, 1, 1],
-    ...               [1, 2, 3]])
-    >>> list(groupby_indices(X))
-    [(array([0, 0, 0]), array([1])), (array([1, 1, 1]), \
-array([3, 4])), (array([1, 2, 3]), array([0, 2, 5]))]
-    """
-    assert X.ndim == 2
-
-    # Code was modified from this answer:
-    # https://stackoverflow.com/questions/30003068/how-to-get-a-list-of-all-indices-of-repeated-elements-in-a-numpy-array
-    idx_sort = np.lexsort(X.T[::-1, :], axis=0)
-    sorted_X = X[idx_sort, :]
-    vals, idx_start = np.unique(sorted_X, return_index=True, axis=0)
-    res = np.split(idx_sort, idx_start[1:])
-    yield from zip(vals, res)
-
-
 class AdaptiveESMDA(BaseESMDA):
     @staticmethod
     def correlation_threshold(ensemble_size: int) -> float:
@@ -294,36 +257,30 @@ def correlation_threshold(ensemble_size: int) -> float:
             assert cov_YY.ndim == 2, "'cov_YY' must be a 2D array"
             assert cov_YY.shape == (Y.shape[0], Y.shape[0])
 
-        for unique_row, indices_of_row in groupby_indices(significant_corr_XY):
-            if verbose:
-                print(
-                    f"    Assimilating {len(indices_of_row)} parameters"
-                    + " with identical correlation thresholds to responses."
-                )
-                print(
-                    "    The parameters are significant wrt "
-                    + f"{np.sum(unique_row)} / {len(unique_row)} responses."
-                )
+        # Identify rows with at least one significant correlation.
+        significant_rows = np.any(significant_corr_XY, axis=1)
 
-            # These parameters are not significantly correlated to any responses
-            if np.all(~unique_row):
-                continue
+        # Loop only over rows with significant correlations
+        for param_num in np.where(significant_rows)[0]:
+            correlated_responses = significant_corr_XY[param_num]
 
-            Y_subset = Y[unique_row, :]
+            Y_subset = Y[correlated_responses, :]
 
             # Compute the masked arrays for these variables
-            cov_XY_mask = np.ix_(indices_of_row, unique_row)
+            cov_XY_mask = np.ix_([param_num], correlated_responses)
             cov_XY_subset = cov_XY[cov_XY_mask]
 
-            cov_YY_mask = np.ix_(unique_row, unique_row)
+            cov_YY_mask = np.ix_(correlated_responses, correlated_responses)
             cov_YY_subset = cov_YY[cov_YY_mask]
 
             # Slice the covariance matrix
             C_D_subset = (
-                self.C_D[unique_row] if self.C_D.ndim == 1 else self.C_D[cov_YY_mask]
+                self.C_D[correlated_responses]
+                if self.C_D.ndim == 1
+                else self.C_D[cov_YY_mask]
             )
 
-            D_subset = D[unique_row, :]
+            D_subset = D[correlated_responses, :]
 
             # Compute transition matrix T
             T = self.compute_cross_covariance_multiplier(
@@ -333,7 +290,7 @@ def correlation_threshold(ensemble_size: int) -> float:
                 Y=Y_subset,
                 cov_YY=cov_YY_subset,  # Passing cov(Y, Y) avoids re-computation
             )
-            X[indices_of_row, :] += cov_XY_subset @ T
+            X[[param_num], :] += cov_XY_subset @ T
 
         return X
 
diff --git a/tests/test_experimental.py b/tests/test_experimental.py
index 39e36ec8..73565ff7 100644
--- a/tests/test_experimental.py
+++ b/tests/test_experimental.py
@@ -1,4 +1,3 @@
-import functools
 import time
 from copy import deepcopy
 
@@ -13,39 +12,9 @@
 from iterative_ensemble_smoother.experimental import (
     AdaptiveESMDA,
     ensemble_smoother_update_step_row_scaling,
-    groupby_indices,
 )
 
 
-@pytest.mark.parametrize("seed", range(25))
-def test_groupby_indices(seed):
-    rng = np.random.default_rng(seed)
-    rows = rng.integers(10, 100)
-    columns = rng.integers(2, 9)
-
-    # Create data matrix
-    X = rng.integers(0, 10, size=(rows, columns))
-
-    groups = list(groupby_indices(X))
-    indices = [set(idx) for (_, idx) in groups]
-
-    # Verify that every row is included
-    union_idx = functools.reduce(set.union, indices)
-    assert union_idx == set(range(rows))
-
-    # Verify that no duplicate rows occur
-    intersection_idx = functools.reduce(set.intersection, indices)
-    assert intersection_idx == set()
-
-    # Verify each entry in the groups
-    for unique_row, indices_of_row in groups:
-        # Repeat this unique row the number of times it occurs in X
-        repeated = np.repeat(
-            unique_row[np.newaxis, :], repeats=len(indices_of_row), axis=0
-        )
-        assert np.allclose(X[indices_of_row, :], repeated)
-
-
 @pytest.fixture()
 def linear_problem(request):
     # Seed the problem using indirect parametrization: