## 5 Evaluation and Translation Results
### Experiment Results and Discussion
\begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline \multirow{3}{*}{**Method**} & \multicolumn{8}{c}{**Chinese\(\rightarrow\)Japanese**} \\ \cline{2-9}  & \# Sentences & \# Back-Translated & \multicolumn{1}{c}{**Ppl**} & \multicolumn{1}{c}{**BLEU (\%)**} & \multicolumn{1}{c}{**TER (\%)**} \\ \cline{2-9}  & **Raw** & **Used** & & **Dev** & **Dev-Test** & **Test** & **Dev-Test** & **Test** \\ \hline Baseline 1 & 300k & 300k & 0 & 3.6 & 38.5 & 38.7 & 44.0 & 44.8 \\ Baseline 2 for P1 & 518k & 518k & 218k & 3.5 & 39.1 & 39.4 & 43.9 & 45.3 \\ Baseline 2 for P2 & 531k & 531k & 231k & 3.5 & 39.2 & 39.5 & 43.0 & 43.8 \\ Copied & 977k & 977k & 0 & 3.5 & 38.9 & 39.2 & 43.7 & 43.9 \\ Partial & 984k & 984k & 0 & 3.5 & 38.9 & 39.2 & 43.0 & 43.4 \\ Proposed 1 & 952k & 923k & 218k & 3.4 & 39.2 & 39.5 & 44.2 & 43.4 \\ Proposed 2 & 977k & 945k & 231k & 3.4 & 39.3 & 39.7 & 42.9 & 43.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Experiment results of Japanese\(\rightarrow\)Chinese Machine Translation (MT) with 300k training data. “ppl” denotes perplexity. “Dev” denotes development data. “Dev-test” denotes development-test data. \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline \multirow{3}{*}{**Method**} & \multicolumn{8}{c}{**Chinese\(\rightarrow\)Japanese**} \\ \cline{2-9}  & \# Sentences & \# Back-Translated & \multicolumn{1}{c}{**Ppl**} & \multicolumn{1}{c}{**BLEU (\%)**} & \multicolumn{1}{c}{**TER (\%)**} \\ \cline{2-9}  & **Raw** & **Used** & & **Dev** & **Dev-Test** & **Test** & **Dev-Test** & **Test** \\ \hline Baseline 1 & 300k & 300k & 0 & 2.7 & 38.1 & 38.0 & 44.9 & 44.8 \\ Baseline 2 for P1 & 518k & 518k & 218k & 2.6 & 40.0 & 39.4 & 44.5 & 44.1 \\ Baseline 2 for P2 & 529k & 529k & 229k & 2.6 & 40.1 & 39.5 & 43.6 & 43.8 \\ Copied & 972k & 972k & 0 & 2.6 & 39.7 & 39.8 & 43.5 & 43.3 \\ Partial & 984k & 984k & 0 & 2.6 & 39.0 & 39.2 & 43.9 & 43.8 \\ Proposed 1 & 952k & 947k & 218k & 2.5 & 40.2 & 40.1 & 42.9 & 43.3 \\ Proposed 2 & 972k & 967k & 229k & 2.5 & 40.5 & 40.2 & 43.6 & 43.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Experiment results of Chinese\(\rightarrow\)Japanese MT with 300k training data. “ppl” denotes perplexity. “Dev” denotes development data. “Dev-test” denotes development-test data. generated pseudo-parallel sentences pairs and unchanged sentences pairs. This method expanded the parallel corpus from 300k sentence pairs to 977k and 972k sentence pairs in each direction. \"Partial\" was the method that augmented the corpus with parallel partial sentences generated by the procedure in Section 4.3, without back-translating and mixing the partial sentences. The experiment of this method aimed to confirm that the mixing step (Section 4.3, Step 2) was necessary. This method expanded the parallel corpus from 300k sentence pairs to 984k sentence pairs in both directions. \"# sentences\" in the tables denotes the size (the number of sentence pairs) of training data, whereas \"# back-translated\" denotes the number of parallel sentence pairs used for back-translation processing, i.e., the corpus augmentation, in each method. \"ppl\" denotes the best (lowest) perplexity values on the development (dev) data in each method. In the case of 300k training data, the number of parallel sentence pairs augmented by Proposed 2 was 977k in the Japanese\(\rightarrow\)Chinese direction. However, only 945k pairs were used as the training data. This is because the translation errors in the back-translation steps sometimes generated unusually long pseudo-source sentences where the same words or phrases occurred repeatedly in a sentence, and such sentence pairs were removed due to exceeding the upper limit of the training data length (500 characters). As a result, the used training data size of Proposed 2 was 32k (3.3%) smaller than that of \"Copied\" (977k). For this reason, the training sentence numbers of \"Copied\" and \"Proposed 2\" in Tables 4 and 5 are different.