diff --git a/dlib/statistics/statistics_abstract.h b/dlib/statistics/statistics_abstract.h
index 8fbc37cd4..36b8a26c1 100644
--- a/dlib/statistics/statistics_abstract.h
+++ b/dlib/statistics/statistics_abstract.h
@@ -153,7 +153,7 @@ namespace dlib
             requires
                 - current_n() > 1
             ensures
-                - return (val-mean())/std::sqrt(variance());
+                - return (val-mean())/stddev();
         !*/
     };