diff --git a/src/lib/sgd.h b/src/lib/sgd.h
index 06dcc46..a3b64a9 100644
--- a/src/lib/sgd.h
+++ b/src/lib/sgd.h
@@ -140,7 +140,7 @@ typedef struct
  * @param    x          Pointer of the solution.
  * @param    g          Pointer of the model gradient.
  * @param    n_size     Length of the solution.
- * @param    m          Length of the observation.
+ * @param    m          Index of the observation.
  * 
  * @return   Value of objective function.
  */