I want to save data for each group during subsession, So I define a new class named Data, and use Data.xxxx to store my group data. I wonder is it a safe way to do so? Do I need to reset Data.xxxx manually if a new session start? 

For example, one of datas I want to store is group_matrix, so some subsessions can share it. I use Data.group_matrix to store it(like I describe above), do I need to reset it before another session start?