  Automatic evaluation on low-resource language translation suffers from a
deficiency of parallel corpora. Round-trip translation could be served as a
clever and straightforward technique to alleviate the requirement of the
parallel evaluation corpus. However, there was an observation of obscure
correlations between the evaluation scores by forward and round-trip
translations in the era of statistical machine translation (SMT). In this
paper, we report the surprising finding that round-trip translation can be used
for automatic evaluation without the references. Firstly, our revisit on the
round-trip translation in SMT evaluation unveils that its long-standing
misunderstanding is essentially caused by copying mechanism. After removing
copying mechanism in SMT, round-trip translation scores can appropriately
reflect the forward translation performance. Then, we demonstrate the
rectification is overdue as round-trip translation could benefit multiple
machine translation evaluation tasks. To be more specific, round-trip
translation could be used i) to predict corresponding forward translation
scores; ii) to improve the performance of the recently advanced quality
estimation model; and iii) to identify adversarial competitors in shared tasks
via cross-system verification.
