## 5 Evaluation and Translation Results
### Experiment Results and Discussion

Table 6: Experiment results of Japanese\(\rightarrow\)Chinese MT with 150k sentences and 522k monolingual sentences. \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline \multirow{3}{*}{**Method**} & \multicolumn{6}{c}{**Chinese\(\rightarrow\)Japanese**} \\ \cline{2-9}  & \multicolumn{2}{c}{**\# Sentences**} & \multirow{2}{*}{**\# Back-Translated**} & \multirow{2}{*}{**ppl**} & \multicolumn{2}{c}{**BLEU (\%)**} & \multicolumn{2}{c}{**TER (\%)**} \\ \cline{2-9}  & **Raw** & **Used** & & & **Dev** & **Dev-Test** & **Test** & **Dev-Test** & **Test** \\ \hline Baseline 1 & 150k & 150k & 0 & 3.1 & 35.4 & 35.5 & 48.4 & 47.5 \\ Baseline 2 + mono (522k) & 672k & 672k & 522k & 2.8 & 39.3 & 39.1 & 45.1 & 44.6 \\
150k + mono (522k) + P2 & 2239k & 2134k & 515k & 2.7 & 40.6 & 40.1 & 43.7 & 43.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Experiment results of Chinese\(\rightarrow\)Japanese MT with 150k sentences and 522k monolingual sentences. “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{6}{c}{**Chinese\(\rightarrow\)Japanese**} \\ \cline{2-9}  & \multicolumn{2}{c}{**\# Sentences**} & \multirow{2}{*}{**\# Back-Translated**} & \multirow{2}{*}{**ppl**} & \multicolumn{2}{c}{**BLEU (\%)**} & \multicolumn{2}{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 + mono (372k) & 672k & 672k & 372k & 3.4 & 39.6 & 39.7 & 42.8 & 44.2 \\
300k + mono (372k) + P2 & 2287k & 2234k & 522k & 3.4 & 39.8 & 40.1 & 42.6 & 43.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Experiment results of Japanese\(\rightarrow\)Chinese MT with 300k sentences and 372k monolingual sentences. “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{6}{c}{**Chinese\(\rightarrow\)Japanese**} \\ \cline{2-9}  & \multicolumn{2}{c}{**\# Sentences**} & \multirow{2}{*}{**\# Back-Translated**} & \multirow{2}{*}{**ppl**} & \multicolumn{2}{c}{**BLEU (\%)**} & \multicolumn{2}{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 + mono (372k) & 672k & 672k & 372k & 2.6 & 40.5 & 39.9 & 43.3 & 43.3 \\
300k + mono (372k) + P2 & 2223k & 2213k & 501k & 2.5 & 41.8 & 41.4 & 42.2 & 42.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Experiment results of Chinese\(\rightarrow\)Japanese MT with 300k sentences and 372k monolingual sentences. “ppl” denotes perplexity. “Dev” denotes development data. “Dev-test” denotes development-test data. expanded the parallel corpus from 150k sentence pairs to 672k sentence pairs. The experiment of this method aimed to confirm the effectiveness of applying our proposed methods to the augmented data by the back-translation method of [PERSON] et al. [11]. \"150k + mono (522k) + P2\" represents the combination method of \"Baseline 2 + mono (522k)\" (672k training data) and \"Proposed 2\". \"150k + mono (522k) + P2\" back-translated 525k and 515k from the \"Baseline 2 + mono (522k)\" (672k training data), so that the numbers of sentence pairs were increased from 672k to 2313k and 2239k in both directions. For Tables 8 and 9, \"Baseline 1\" is a character-level translation which did not process anything with 300k original training data. The back-translation models were constructed using 300k original training data of Baseline 1 for each method, before corpus augmentation. The back-translation models were constructed using 300k original training data of \"Baseline 1\" for each method, before corpus augmentation. \"Baseline 2 + mono (372k)\" was the back-translation method of [PERSON] et al.