'The derivative of Tukey\'s biweight psi function
Notes
Used to estimate the robust covariance matrix.'

def psi_deriv(self, z):

    subset = self._subset(z)
    return (subset * (((1 - ((z / self.c) ** 2)) ** 2) - (((4 * (z ** 2)) / (self.c ** 2)) * (1 - ((z / self.c) ** 2)))))
