The quantitatively different response of these competing defect species to supervalent or subvalent doping can be characterised in terms of a "doping-response efficiency", η X , which we define as the change in concentration of a defect species, ∆[X], for a fixed effective dopant concentration, r [M r ], divided by the change in concentration if the doping response were purely due to defect X in charge state q , i.e.,