Creates model latent variables

        Returns
        ----------
        None (changes model attributes)

def _create_latent_variables(self):
        """ Creates model latent variables

        Returns
        ----------
        None (changes model attributes)
        """

        self.latent_variables.add_z('Constant', fam.Normal(0,10,transform=None), fam.Normal(0,3))
        self.latent_variables.add_z('Ability Scale', fam.Normal(0,1,transform=None), fam.Normal(0,3))