การคำนวณตัวแปรแบบรวม

# Maximum of donations for each donor in december 2016
gifts_max_december_2016 = gifts_december_2016.____("donor_id")["amount"].____().reset_index()
gifts_max_december_2016.columns = ["donor_id", "max_amount"]

# Maximum of donations for each donor in december 2017
gifts_max_december_2017 = gifts_december_2017.____("donor_id")["amount"].____().reset_index()
gifts_max_december_2017.columns = ["donor_id", "max_amount"]