'The robust criterion function for Tukey\'s biweight estimator
Parameters
z : array-like
1d array
Returns
rho : array
rho(z) = -(1 - (z/c)**2)**3 * c**2/6.   for \|z\| <= R
rho(z) = 0                              for \|z\| > R'

def rho(self, z):

    subset = self._subset(z)
    return ((((- ((1 - ((z / self.c) ** 2)) ** 3)) * subset) * (self.c ** 2)) / 6.0)
