We can observe a noticeable gap between unsupervised and supervised methods, but the gap is narrowing as the rank increases.
unsupervised NMT The current NMT systems (Sutskever et al., 2014;Cho et al., 2014a;Bahdanau et al., 2015;Gehring et al., 2017;Vaswani et al., 2017) are known to easily overfit and result in an inferior performance when the training data is limited (Koehn and Knowles, 2017;Isabelle et al., 2017;Sennrich, 2017).