diff --git a/markdown/Ch9. Specification and Data Issues.md b/markdown/Ch9. Specification and Data Issues.md
index 0b71315..d62ccd1 100644
--- a/markdown/Ch9. Specification and Data Issues.md	
+++ b/markdown/Ch9. Specification and Data Issues.md	
@@ -13,7 +13,7 @@ jupyter:
     name: python3
 ---
 
-# Ch9. Specification and Data Issues
+# 9. Specification and Data Issues
 
 ```python
 %pip install matplotlib numpy pandas statsmodels wooldridge scipy -q
diff --git a/notebooks/Ch9. Specification and Data Issues.ipynb b/notebooks/Ch9. Specification and Data Issues.ipynb
index 3159a13..4c0a5d1 100644
--- a/notebooks/Ch9. Specification and Data Issues.ipynb	
+++ b/notebooks/Ch9. Specification and Data Issues.ipynb	
@@ -5,7 +5,7 @@
    "id": "5d3db9d2",
    "metadata": {},
    "source": [
-    "# Ch9. Specification and Data Issues"
+    "# 9. Specification and Data Issues"
    ]
   },
   {
diff --git a/scripts/Ch9. Specification and Data Issues.py b/scripts/Ch9. Specification and Data Issues.py
index b73c94a..e02ef87 100644
--- a/scripts/Ch9. Specification and Data Issues.py	
+++ b/scripts/Ch9. Specification and Data Issues.py	
@@ -13,7 +13,7 @@
 #     name: python3
 # ---
 
-# # Ch9. Specification and Data Issues
+# # 9. Specification and Data Issues
 
 # %pip install matplotlib numpy pandas statsmodels wooldridge scipy -q