diff --git a/lib/iris/analysis/stats.py b/lib/iris/analysis/stats.py
index 9654cdea0f..3eda09c624 100644
--- a/lib/iris/analysis/stats.py
+++ b/lib/iris/analysis/stats.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2013 - 2015, Met Office
+# (C) British Crown Copyright 2013 - 2019, Met Office
 #
 # This file is part of Iris.
 #
@@ -31,6 +31,8 @@
 def _ones_like(cube):
     """
     Return a copy of cube with the same mask, but all data values set to 1.
+
+    The operation is non-lazy.
     """
     ones_cube = cube.copy()
     ones_cube.data = np.ones_like(cube.data)
@@ -85,6 +87,8 @@ def pearsonr(cube_a, cube_b, corr_coords=None, weights=None, mdtol=1.,
     Reference:
         http://www.statsoft.com/textbook/glosp.html#Pearson%20Correlation
 
+    This operation is non-lazy.
+
     """
 
     # Assign larger cube to cube_1