'Derivative of CDF link
Parameters
p : array-like
mean parameters
Returns
g\'(p) : array
The derivative of CDF transform at `p`
Notes
g\'(`p`) = 1./ `dbn`.pdf(`dbn`.ppf(`p`))'

def deriv(self, p):

    p = self._clean(p)
    return (1.0 / self.dbn.pdf(self.dbn.ppf(p)))
