calculates the estimated happiness of a person
        living in a world
        self._update_pref(self.person.prefs['tax_min'], self.person.prefs['tax_max'], self.world.tax_rate)
        self._update_pref(self.person.prefs['tradition'], self.person.prefs['tradition'], self.world.tradition)
        self._update_pref(self.person.prefs['equity'], self.person.prefs['equity'], self.world.equity)

def calculate(self):
        """
        calculates the estimated happiness of a person
        living in a world
        self._update_pref(self.person.prefs['tax_min'], self.person.prefs['tax_max'], self.world.tax_rate)
        self._update_pref(self.person.prefs['tradition'], self.person.prefs['tradition'], self.world.tradition)
        self._update_pref(self.person.prefs['equity'], self.person.prefs['equity'], self.world.equity)
        """
        self.rating = 0
        for f in self.factors:
            self._update_pref(f.min, f.max, self.world.tax_rate)