diff --git a/htdocs/core/class/stats.class.php b/htdocs/core/class/stats.class.php
index 49a2c6d4403..63125fffd9f 100644
--- a/htdocs/core/class/stats.class.php
+++ b/htdocs/core/class/stats.class.php
@@ -233,6 +233,12 @@ abstract class Stats
 		return $data;
 	}
 
+	/**
+	 * @param	int     $year           year number
+	 * @return 	int						value
+	 */
+	protected abstract function getAverageByMonth($year);
+
 	/**
 	 * Return average of entity by month for several years
 	 *