diff --git a/04-One-Dimensional-Kalman-Filters.ipynb b/04-One-Dimensional-Kalman-Filters.ipynb
index 35a75c5..0a646d4 100644
--- a/04-One-Dimensional-Kalman-Filters.ipynb
+++ b/04-One-Dimensional-Kalman-Filters.ipynb
@@ -722,7 +722,7 @@
     "x = gaussian(0., 20.**2)  # dog's position, N(0, 20**2)\n",
     "velocity = 1\n",
     "dt = 1. # time step in seconds\n",
-    "process_model = gaussian(velocity, process_var) \n",
+    "process_model = gaussian(velocity*dt, process_var) # displacement to add to x\n",
     "  \n",
     "# simulate dog and get measurements\n",
     "dog = DogSimulation(\n",