The only exception is translation into Finnish, in which perplexity on the PBMT output is slightly lower, probably because its fluency was improved by reranking it with a neural LM similar to the one 7 http://data.statmt.org/ wmt16/translation-task/ training-monolingual-news-crawl.tgz  .
our experiment shows that the outputs produced by NMT systems are, in general, more fluent than those produced by PBMT systems.