--- a
+++ b/tests/utils/test_splines.py
@@ -0,0 +1,50 @@
+import unittest
+
+import numpy as np
+
+from inmoose.utils import ns, spline_design
+
+
+class Test(unittest.TestCase):
+    def test_splineDesign(self):
+        spline = spline_design(np.arange(10), [4.5, 5, 5.5], 4)
+        self.assertTrue(
+            np.allclose(
+                spline,
+                np.array(
+                    [
+                        [0, 0.02083333, 0.47916667, 0.4791667, 0.02083333, 0.0],
+                        [0, 0.00000000, 0.16666667, 0.6666667, 0.16666667, 0.00000000],
+                        [0, 0.00000000, 0.02083333, 0.4791667, 0.47916667, 0.02083333],
+                    ]
+                ),
+            )
+        )
+
+    def test_splines(self):
+        res = ns([1, 2, 3, 4, 5, 6], df=3, include_intercept=True)
+        self.assertTrue(
+            np.allclose(
+                res.basis,
+                [
+                    [-0.45528153, 0.5915984, -0.39439896],
+                    [0.04538466, 0.4796729, -0.30911524],
+                    [0.39866939, 0.3936580, -0.17710530],
+                    [0.46333206, 0.3583848, 0.04641015],
+                    [0.23937270, 0.3738533, 0.36143111],
+                    [-0.13196814, 0.4152326, 0.72317830],
+                ],
+            )
+        )
+
+        res2 = res.predict(newx=[70, 81, 92])
+        self.assertTrue(
+            np.allclose(
+                res2.basis,
+                [
+                    [-25.46985, 3.339883, 24.37341],
+                    [-29.82480, 3.842558, 28.43829],
+                    [-34.17975, 4.345232, 32.50318],
+                ],
+            )
+        )