# Phrase Table Combination Based on Symmetrization of Word Alignment for Low-Resource Languages
## 4 Results and Discussion
### Interpolation System Approach (ISA)
The result demonstrated that relatively similar phrase translation parameter scores using different symmetrization of word alignment could obtain the same BLEU score. We investigated the relationship between BLEU score and phrase table size in each system, as shown in Tables 7 and 9. We found that systems with small phrase tables, i.e., Kk-En LM03 of H-ISA, Kk-En LM05 of H-ISA, and Ja-Id LM05 of baseline, obtained higher BLEU scores: 3.43, 3.64, and 12.20, respectively, as shown in Table 7. However, we also found that systems with large phrase tables, i.e., Ja-Id LM03 of Std-ISA and Ja-Id LM03 of H-ISA, obtained higher BLEU scores, that is, 12.07 and 12.07, respectively. Our first finding aligns with [38], who stated that higher BLEU scores could be obtained when using small phrase tables. Our second finding aligns with that of [11], who obtained higher BLEU scores when using large phrase tables. Our results demonstrated that higher BLEU scores can be obtained when using either small or large phrase tables. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Language Model** & **Baseline** & **Std-ISA** & **H-ISA** \\ \hline Kk–En & & & \\ LM03 & 3.08 & 3.08 & **3.43** \\ LM05 & 3.42 & 3.42 & **3.64** \\ Ja–Id & & & \\ LM03 & 11.96 & **12.07** & **12.07** \\ LM05 & **12.20** & 12.08 & 12.08 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The obtained BLEU scores of direct translation of src–trg (baseline) and interpolation system approach (ISA). Results in bold indicate the highest translation quality. \begin{table}
\begin{tabular}{c l c c c} \hline \hline \multirow{2}{*}{**Phrase-Pair**} & \multirow{2}{*}{**Phrase Translation Parameter**} & \multicolumn{3}{c}{**Phrase Table**} \\ \cline{3-5}  & & **Baseline** & **Std-ISA** & **H-ISA** \\ \hline  & Inverse phrase translation probability (\(p(t|s)\)) & 9.52381e-3 & _8.41842e-3_ & _8.49199e-3_ \\ \(\sum\)\(\partial  
We identified why systems with small or large phrase tables could obtain higher BLEU scores when using the same LM order and decoder weights. We compared phrase translation parameter scores between the two phrase tables of the highest and second-highest BLEU score in the same LM. First, we collected 2000 phrase pairs and their phrase translation parameter scores from the phrase table of the highest BLEU score as phrase Table 1 (PT1). Subsequently, we collected 12,000 phrase pairs and their phrase translation parameter scores from the phrase table of second-highest BLEU score as phrase Table 2 (PT2). Lastly, we examined whether the phrase translation parameter of PT1 obtains higher scores than that of PT2 in the same phrase pair. We found that the phrase translation parameter of PT1 obtained higher scores than that of PT2 in a system with a small phrase table, as shown in Table 10. Consider an example of Kk-En LM03 of H-ISA that obtained higher scores, 514, 576, and 639, in \(lex(t|s)\), \(p(s|t)\) and \(lex(s|t)\), respectively, compared to Kk-En LM03 using the Std-ISA. The result indicated that a system with a small phrase table could obtain a higher BLEU score because of the higher phrase translation parameter scores, particularly in \(p(s|t)\) and \(lex(s|t)\). In contrast to the system with a small phrase table, we found that the phrase translation parameter of PT1 had a lower score than that of PT2 in a system with a large phrase table. Table 10 shows that Ja-Id LM03 had lower scores, 13, 18, 17, and 34, in \(p(t|s)\), \(lex(t|s)\), \(p(s|t)\), and \(lex(s|t)\), respectively. The result demonstrated that a system with a large phrase table can obtain higher BLEU scores due to the lower phrase translation parameter scores. Table 11 shows the phrase pair and its phrase translation parameter score examples from Ja-Id LM05. We evaluated the perplexity score of each system, as shown in Table 12. We found that the longer LM order of Kk-En, i.e., LM05, obtained a lower perplexity score than the shorter one, i.e., LM03. In contrast, the longer LM order could not obtain a lower perplexity score than the shorter one for Ja-Id.