# Phrase Table Combination Based on Symmetrization of Word Alignment for Low-Resource Languages
## 4 Results and Discussion
### Interpolation System Approach (ISA)  
In the ISA, we constructed two subsystems: Std-ISA and H-ISA. Std-ISA was our interpolation system that uses gdfand, whereas H-ISA uses the symmetrization of word alignment that obtained the highest BLEU score. The choice of symmetrization of word alignment for H-ISA is shown in Table 6. Considering Kk-En LM05 H-ISA as an example, we employed tgttosrc in Kk-En as src-trg, whereas we used gdfand in Kk-Ru as src-pvt and Ru-En as pvt-trg. Table 7 shows the ISA result. We included the direct translation src-trg of Kk-En and Ja-Id as a baseline. We found that all the translation systems using LM05 obtained higher BLEU scores than those using LM03. For Kk-En, we found that H-ISA is a competitive approach because it provided absolute improvements of 0.35 and 0.22 BLEU points over baseline and Std-ISA in LM03 and LM05, respectively. Table 7 shows the different effect of H-ISA on Ja-Id. H-ISA obtained absolute improvements of 0.11 BLEU points over baseline in LM03. However, H-ISA obtained an absolute drop of \(-\)0.12 BLEU points compared to baseline in LM05. We compared 2000 of the same phrase pairs from two phrase tables: H-ISA and baseline. We provide an example of H-ISA and baseline phrase pairs and phrase translation parameter score in Table 8. We found that more than 1900 phrase pairs of H-ISA in LM05 obtained lower phrase translation parameter scores compared to baseline in LM05. Therefore, lower phrase translation parameter scores could be a reason for the lower BLEU score for Ja-Id using the H-ISA. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline  & **Kk–En** & \multicolumn{4}{c}{**Ja–Id**} \\ \hline
**LM Order** & **Lang Pair** & **H-ISA** & **LM Order** & **Lang Pair** & **H-ISA** \\ \hline LM03 & Kk–En & tgttosrc & LM03 & Ja–Id & gdfand \\  & Kk–Ru & gdfand & & Ja-Ms & gdfand \\  & Ru–En & gdfand & & Ms-Id & gdfand \\ LM05 & Kk–En & tgttosrc & LM05 & Ja–Id & gdfand \\  & Kk–Ru & gdfand & & Ja-Ms & gdfand \\  & Ru–En & gdfand & & Ms-Id & tgttosrc \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Candidates of symmetrization of word alignment for highest-interpolation system approach (H-ISA). \begin{table}
\begin{tabular}{l l l l} \hline \hline \multicolumn{1}{c}{**Phrase Pair**} & \multicolumn{1}{c}{**Phrase Translation Parameters**} & \multicolumn{2}{c}{**Symmetrization**} \\ \cline{3-4} \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} \\ \cline{3-4} \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} \\ \cline{3-4} \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Example of phrase translation parameter scores in Kk–En LM03. Results in bold indicate the highest scores. Additionally, we investigated why Ja-Id LM03 using the Std-ISA and Ja-Id LM03 using the H-ISA obtained same BLEU score: 12.07 and 12.07, respectively. We found that both systems used the same candidates of symmetrization of word alignment in three sides of the pivot approaches: gdfand in Ja-Id, gdfand in Ja-Ms, and gdfand in Ms-Id, as shown in Table 6. In contrast to Ja-Id LM03, Ja-Id LM05 with the Std-ISA and Ja-Id LM05 with the H-ISA obtained the same BLEU score when using different candidates of symmetrization of word alignment, as shown in Table 6. Ja-Id LM05 using the Std-ISA used gdfand in Ja-Id, gdfand in Ja-Ms and gdfand in Ms-Id. Ja-Id LM05 of H-ISA used gdfand in Ja-Id, gdfand in Ja-Ms and tgttosrc in Ms-Id. We compared 2000 of the same phrase pairs from two phrase tables: Std-ISA and H-ISA. Table 8 presents an example of Std-ISA and H-ISA phrase pairs and phrase translation parameter scores. We found that more than 1700 phrase pairs of Std-ISA and H-ISA obtained relatively similar phrase translation parameter scores.