def affiche(self):

        for key,value in self.tab.items():

            print(key,":",value)



    def moyenne(self):

        m,s = 0,0

        for key,value in self.tab.items():

            m += key * value

            s += value

        return m / s



    def var(self):

        R , F = [] , []

        m = self.moyenne()



        for key,value in self.tab.items():

            R.append( (key - m)**2 )

            F.append( value )



        return SerieStat(R,F).moyenne()



    def ecart(self):

        return self.var()**0.5