You may have noticed a performance change on a production system and would like to know why, or you may have implemented a change to a production system and want to know the effect of the change, such as increased concurrency waits.
The Compare Period ADDM compares the performance of the database server in two time periods, and returns a report describing the performance changes and the root origin of the changes
http://docs.oracle.com/database/121/TDPPT/tdppt_degrade.htm#TDPPT525