diff --git a/ch03/sigmoid_test.py b/ch03/sigmoid_test.py
new file mode 100644
index 00000000..d59021ed
--- /dev/null
+++ b/ch03/sigmoid_test.py
@@ -0,0 +1,21 @@
+import matplotlib.pyplot as plt
+import numpy as np
+
+
+def sigmoid(z):
+    return 1.0 / (1.0 + np.exp(-z))
+
+
+z = np.arange(-7, 7, 0.1)
+phi_z = sigmoid(z)
+plt.plot(z, phi_z)
+plt.axvline(0.0, color="k")
+plt.ylim(-0.1, 1.1)
+plt.xlabel("z")
+plt.ylabel("$\phi (z)$")
+plt.yticks([0.0, 0.5, 1.0])
+ax = plt.gca()
+ax.yaxis.grid(True)
+plt.tight_layout()
+
+plt.show()