This Recommendation1 describes the methodology for deriving equipment impairment factors (Ies) using instrumental models. The equipment impairment factors derived by this methodology are intended to be used in the E-model (see [ITU-T G.107]). The methodology is to be considered as supplementary to the one based on auditory listening-only tests described in [ITU-T P.833]. It will provide valid Ie values only for those codecs for which the instrumental model used produces meaningful estimations. 1 This Recommendation includes an electronic attachment containing speech material.