code: def ets(self):
          
          r = (self.table[0, 0] + self.table[0, 1]) * (self.table[0, 0] + self.table[1, 0]) / self.N
          return (self.table[0, 0] - r) / (self.table[0, 0] + self.table[0, 1] + self.table[1, 0] - r)
docs: Equitable Threat Score, Gilbert Skill Score, v, (a - R)/(a + b + c - R), R=(a+b)(a+c)/N