diff --git a/interp/tools.py b/interp/tools.py
index e5e2f75..a86f9ba 100644
--- a/interp/tools.py
+++ b/interp/tools.py
@@ -68,4 +68,4 @@ def improved_answer(answer, exact):
 
 
 def percent_improvement(answer, exact):
-  return np.abs(answer['qlin'] - exact) / exact
+  return np.abs(answer['error']) / exact