diff --git a/wosis/analysis/stats.py b/wosis/analysis/stats.py
index 95b5d5c..955bbd4 100644
--- a/wosis/analysis/stats.py
+++ b/wosis/analysis/stats.py
@@ -2,9 +2,9 @@
 
 def calc_average_citations(in_df, max_year=None, sort=False):
     """Calculate the average citations since year of publication.
-	
-	If no `max_year` is specified, calculate using the latest 
-	year in given dataset.
+    
+    If no `max_year` is specified, calculate using the latest 
+    year in given dataset.
 
     Example
     ==========
@@ -18,7 +18,7 @@ def calc_average_citations(in_df, max_year=None, sort=False):
     Parameters
     ==========
     * in_df : Pandas DataFrame
-	* max_year: int or None, year to calculate average citations from.
+    * max_year: int or None, year to calculate average citations from.
 
     Returns
     ==========
@@ -27,15 +27,15 @@ def calc_average_citations(in_df, max_year=None, sort=False):
     assert hasattr(in_df, 'citations'), \
         'DataFrame has to have `citation` column. Use `get_num_citations()` first'
     out_df = in_df.copy()
-	
-	max_year_in_data = out_df.year.max()
-	
-	if max_year not None:
-		max_year = int(max_year)
-		assert max_year_in_data <= max_year, \
-			"Given max_year must be later than any year found in dataset."
+    
+    max_year_in_data = out_df.year.max()
+    
+    if max_year not None:
+        max_year = int(max_year)
+        assert max_year_in_data <= max_year, \
+            "Given max_year must be later than any year found in dataset."
     else:
-		max_year = out_df.year.max()
+        max_year = out_df.year.max()
     out_df.loc[:, 'Avg. Citations'] = (out_df.citations / ((max_year - out_df.year) + 1)).astype(float).round(2)
 
     if sort: