diff --git a/lecture_4.ipynb b/lecture_4.ipynb
index 296a5577..1841e1d9 100644
--- a/lecture_4.ipynb
+++ b/lecture_4.ipynb
@@ -70,7 +70,50 @@
     }
    },
    "source": [
-    "As an example, we'll look at the frequency of 311 requests:\n",
+    "Simple time series: The population data."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "import pandas as pd\n",
+    "\n",
+    "districts = pd.read_csv('https://storage.googleapis.com/python-public-policy/data/311_community_districts.csv.zip')\n",
+    "districts.head()"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {
+    "slideshow": {
+     "slide_type": "subslide"
+    }
+   },
+   "source": [
+    "Helpful function for looking at changes over time: [`pct_change()`](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.pct_change.html)."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "discticts.pct_change(axis='columns')"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {
+    "slideshow": {
+     "slide_type": "slide"
+    }
+   },
+   "source": [
+    "Next, we'll look at the frequency of 311 requests:\n",
     "\n",
     "- Over time\n",
     "- By day of the week\n",
@@ -106,7 +149,6 @@
    },
    "outputs": [],
    "source": [
-    "import pandas as pd\n",
     "import plotly.express as px\n",
     "\n",
     "# boilerplate for allowing PDF export\n",