This paper compares two approaches where the structure of the eye diagram is considered either as a timing uncertainty or alternatively a vertical uncertainty. We show that only the second view allows accurate BER evaluation, even if the eye is closed. It follows from BER definition that its proper evaluation should be based on considering the two portions of the eye diagram, and by their separate integration. With such understanding of BER, it turns out that the jitter-induced vertical noise can be considered same way as the signal’s ISI, although correlated with the latter.