def _perturbation(self):
        """
        Returns Gaussian perturbation
        """
        if self.P>1:
            scales = []
            for term_i in range(self.n_terms):
                _scales = SP.randn(self.diag[term_i].shape[0])
                if self.offset[term_i]>0:
                    _scales  = SP.concatenate((_scales,SP.zeros(1)))
                scales.append(_scales)
            scales = SP.concatenate(scales)
        else:
            scales = SP.randn(self.vd.getNumberScales())
        return scales
Returns Gaussian perturbation