diff --git a/SimPEG/Survey.py b/SimPEG/Survey.py
index e754476e..42ee50be 100644
--- a/SimPEG/Survey.py
+++ b/SimPEG/Survey.py
@@ -47,6 +47,13 @@ class BaseSurvey(object):
         self.prob._survey = None
         self._prob = None
 
+    @property
+    def nD(self):
+        """Number of data."""
+        if hasattr(self, 'dobs'):
+            return self.dobs.size
+        raise NotImplemented('Number of data is unknown.')
+
     @property
     def ispaired(self): return self.prob is not None
 
diff --git a/docs/api_Forward.rst b/docs/api_Forward.rst
index 067a902f..66d9d36d 100644
--- a/docs/api_Forward.rst
+++ b/docs/api_Forward.rst
@@ -10,10 +10,10 @@ Model
     :undoc-members:
     :inherited-members:
 
-Data
-====
+Survey
+======
 
-.. automodule:: SimPEG.Data
+.. automodule:: SimPEG.Survey
     :show-inheritance:
     :members:
     :undoc-members: