def get_intervar_ranges(theta):
    """ theta (dict): parameters dictionary
    """
    if 'st' in theta.keys():
        theta['t_lo'] = theta['tr'] - theta['st'] / 2
        theta['t_hi'] = theta['tr'] + theta['st'] / 2
    if 'sz' in theta.keys():
        theta['z_lo'] = theta['z'] - theta['sz'] / 2
        theta['z_hi'] = theta['z'] + theta['sz'] / 2
    return theta
