Recently, the present authors proposed a correlated barrier hopping model for bipolarons based on continuous-time random-walk approximation to explain ac conductivity in a-As 2Se 3 and estimated the density of defect states based on this approach. It was observed that the density of defect states deduced from this approach is less than as deduced from correlated barrier hopping based on pair approximation and are consistent with the estimated density of defect states from other studies, namely, light-induced electron spin resonance and drift mobility. Now, this approach is extended to the single- and bi-polaron contribution to ac conductivity and apply to the experimental data of a-Se and a-As 2Se 3 + 0.5 at.% Ag. The present approach explains the single- as well as bi-polaron contribution to ac conductivity and the deduced density of defect states is less than that deduced from correlated barrier hopping based on pair approximation.