diff --git a/synthetic_control.ipynb b/synthetic_control.ipynb
index 4652693..7c2a4f0 100644
--- a/synthetic_control.ipynb
+++ b/synthetic_control.ipynb
@@ -1,5 +1,13 @@
 {
  "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "fb0385d3-2972-4449-9aa7-179afc959dae",
+   "metadata": {},
+   "source": [
+    "# Source: https://matheusfacure.github.io/python-causality-handbook/15-Synthetic-Control.html"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 1,