diff --git a/yli/regress.py b/yli/regress.py
index 5708cc7..5ab7946 100644
--- a/yli/regress.py
+++ b/yli/regress.py
@@ -28,8 +28,16 @@ import itertools
 
 from .utils import Estimate, check_nan, fmt_p_html, fmt_p_text
 
-def vif(df, nan_policy='warn'):
-	"""Calculate the variance inflation factor for each variable in df"""
+def vif(df, formula=None, nan_policy='warn'):
+	"""
+	Calculate the variance inflation factor for each variable in df
+	
+	formula: If specified, calculate the VIF only for the variables in the formula
+	"""
+	
+	if formula:
+		# Only consider columns in the formula
+		df = df[cols_for_formula(formula)]
 	
 	# Check for/clean NaNs
 	df = check_nan(df, nan_policy)