calls_grouped = calls.groupby(['user_id','month']).agg({'duration': ['count', 'sum']}).reset_index()
internet_grouped = internet.groupby(['user_id', 'month']).agg({'mb_used': 'count'}).reset_index()
messages_grouped = messages.groupby(['user_id','month']).agg({'message_date':'count'}).reset_index()