11.2.0.4 on AIX 7.1 : Gathering dictionary , system and fixed tables statistics ?

-- Under which conditions ?                                                                                                                                                       
execute dbms_stats.gather_dictionary_stats();                                                                                                                                               

-- Heavy workload                                                                                                                                     
EXEC DBMS_STATS.GATHER_SYSTEM_STATS('INTERVAL', interval => 15) ;     -- 15 minutes                                                                                                             
-- Fixed tables :                                                                                                                                                                             
--execute dbms_stats.gather_fixed_objects_stats;  

Finally , is it better to delete these statistics before gathering them again ?

Best regards.

GD.