diff --git a/surveytools/__pycache__/scales.cpython-312.pyc b/surveytools/__pycache__/scales.cpython-312.pyc
index 506d524..43132db 100644
Binary files a/surveytools/__pycache__/scales.cpython-312.pyc and b/surveytools/__pycache__/scales.cpython-312.pyc differ
diff --git a/surveytools/scales.py b/surveytools/scales.py
index e0f1756..db708b0 100644
--- a/surveytools/scales.py
+++ b/surveytools/scales.py
@@ -3,7 +3,7 @@
 
 def reverse_score(value, scale_max=5, scale_min=1):
     """Reverse-score one Likert response (e.g. 1 <-> 5 on a 1-5 scale)."""
-    return scale_max - value
+    return (scale_max + scale_min) - value
 
 
 def scale_mean(responses, reverse_items=(), scale_max=5):
@@ -13,8 +13,16 @@ def scale_mean(responses, reverse_items=(), scale_max=5):
     reverse_items: item names to reverse-score before averaging.
     """
     total = 0
+    count = 0
     for item, value in responses.items():
+        # Skip items that were not answered
+        if value is None:
+            continue
         if item in reverse_items:
             value = reverse_score(value, scale_max)
         total += value
-    return total / len(responses)
+        count += 1
+    # If no answered items, return None instead of dividing by zero
+    if count == 0:
+        return None
+    return total / count
diff --git a/tests/test_scales.py b/tests/test_scales.py
index 61f6b03..ea11379 100644
--- a/tests/test_scales.py
+++ b/tests/test_scales.py
@@ -14,3 +14,20 @@ def test_scale_mean_simple():
 def test_scale_mean_with_reverse_item():
     # q2 = 2 reversed is 4, so the mean of 4 and 4 is 4
     assert scale_mean({"q1": 4, "q2": 2}, reverse_items={"q2"}) == 4
+
+
+def test_scale_mean_ignores_none():
+    # q2 is skipped; average of 4 and 2 is 3
+    responses = {"q1": 4, "q2": None, "q3": 2}
+    assert scale_mean(responses) == 3
+
+
+def test_scale_mean_with_reverse_and_none():
+    # q1 is reversed (2 -> 4); q2 is skipped
+    responses = {"q1": 2, "q2": None}
+    assert scale_mean(responses, reverse_items={"q1"}) == 4
+
+
+def test_scale_mean_all_none():
+    # No answered items -> result should be None
+    assert scale_mean({"q1": None, "q2": None}) is None