The authors investigated the problem of overestimation with the Volterra series transfer function (VSTF) and an artificial neural network (ANN), which are used for non-linear equalisers in optical communication systems. The results revealed that the risk of predicting a pseudo-random binary sequence (PRBS) pattern, which causes overestimation of the equaliser performance, occurs not only with an ANN but also with the VSTF. When using PRBS9, PRBS11 and PRBS15, the number of taps of a feedforward tapped delay line, which is required in the VSTF to predict the PRBS pattern, was the same as that with the ANN. When the second-order Volterra kernels were omitted, a larger number of taps was required in the VSTF to observe the overestimation. a EVM versus the number of taps in ANN-based non-linear equaliser b EVM versus the number of taps in VSTF-based non-linear equaliser using first-, second-and third-order Volterra kernels c EVM versus the number of taps in VSTF with first-, second-and third-order Volterra kernels and VSTF with firstand third-order Volterra kernels (the VSTFs were trained on PRBS9) Conclusion: We investigated the problem of overestimation in the ANN-and VSTF-based non-linear equalisers in PRBS-based signal quality evaluation. The results revealed that a VSTF can predict PRBS patterns, and the overestimation problem occurs not only with the ANN but also with the VSTF. In our investigation using PRBS9, PRBS11 and PRBS15, the number of taps that the VSTF required to predict the PRBS pattern was the same as that with the ANN.