International Journal on Natural Language Computing (IJNLC) Vol.11, No.1, February 2022

MORPHOLOGICAL ANALYZER USING THE BILSTM MODEL ONLY FOR JAPANESE HIRAGANASENTENCES
Jun Izutsu1 and Kanako Komiya2
1

2

Ibaraki University

Tokyo University of Agriculture and Technology

ABSTRACTThis study proposes a method to develop neural models of the morphological analyzer for Japanese
Hiragana sentences using the Bi-LSTM CRF model. Morphological analysis is a technique that
divides text data into words and assigns information such as parts of speech. In Japanese natural
language processing systems, this technique plays an essential role in downstream applications
because the Japanese language does not have word delimiters between words. Hiragana is a typeof Japanese phonogramic characters, which is used for texts for children or people who cannot read
Chinese characters. Morphological analysis of Hiragana sentences is more difficult than that ofordinary Japanese sentences because there is less information for dividing. For morphologicalanalysis of Hiragana sentences, we demonstrated the effectiveness of fine-tuning using a model
based on ordinary Japanese text and examined the influence of training data on texts of various
genres.

KEYWORDS
Morphological analysis, Hiragana texts, Bi-LSTM CRF model, Fine-tuning, Domain adaptation

1. INTRODUCTION
1.1. Components of the Japanese Language and the Acquisition Process
Japanese sentences contain various kinds of character, such as Kanji (Chinese character),Hiragana, Katakana, numbers, and alphabet, making it difficult to learn. Japanese
speakers usually learn Hiragana first in their school days because the number of characters
is much smaller than the Kanji; Hiragana has 46 characters, and Japanese use thousandsof Kanji. Most Japanese sentences are composed of all kinds of characters called KanjiKana mixed sentences.
However, it is difficult for many non-Japanese speakers to learn thousands of Kanji, so
children and new Japanese language learners use Hiragana.

DOI: 10.5121/ijnlc.2022.11103

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International Journal on Natural Language Computing (IJNLC) Vol.11, No.1, February 2022

In particular, elementary school students are the first to learn Hiragana and Katakana.
After mastering these two, they learn simple Kanji. Similarly, foreigners who are new toJapanese also learn Hiragana and Katakana first. The first sentences they read are almost
entirely composed of Hiragana.

1.2. Morphological Analysis of Hiragana Sentences
Morphological analysis is a technique that divides natural language text data into words
and assign information such as parts of speech. In Japanese, morphological analysis is oneof the core technologies for natural language processing because the Japanese language
does not have word delimiters between words. Morphological analyzers like MeCab andChasen are now commonly used for morphological analysis. However, since the above
systems target Kanji-Kana mixed sentences, it is challenging to perform the use on
sentences written only in Hiragana.
Morphological analysis of Hiragana-only sentences is more challenging thanmorphological analysis of Kanji-Kana mixed sentences. If the text is a mixture of Kanji
and Kana (Hiragana and Katakana), it will be divisible between Kanji, Hiragana, andKatakana. However, if the text consists only of Hiragana, there will be less information
for dividing. Therefore, we propose to fine-tune the model of Kanji-Kana mixed
sentences and investigated whether the accuracy of morphological analysis of Hiragana
sentences can be improved by inheriting the information to be divided into words.
The Bi-LSTM CRF model was used to develop a morphological analyzer for Hiragana
sentences in this paper. We used two types of training data: Wikipedia and Yahoo!
Answers in the Balanced Corpus of Contemporary Written Japanese, in our studies toinvestigate the influence of the genre of the training data. We also fine-tune both data to
examine the effect of various genres on the text. [7]The following are the four contributions of this paper.
l

Developed a morphological analyzer for Hiragana sentences using the BiLSTM CRF model,lDemonstrated the effectiveness of fine-tuning using a model based on KanjiKana mixed text,l

Examined the influence of training data of morphological analysis on texts ofvarious genres, andlDemonstrated the effectiveness of fine-tuning using data from Wikipedia andYahoo! Answers.

In this paper, we report the results of these experiments. This paper is an extended
version of “Morphological Analysis of Japanese Hiragana Sentences Using the Bi-LSTM
CRF Model”, published in the proceedings of 10th International Conference on Natural
Language Processing (NLP 2021).

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International Journal on Natural Language Computing (IJNLC) Vol.11, No.1, February 2022

2. RELATED WORK
Izutsu et al. (2020) [3] converted MeCabʼs ipadic dictionary into Hiragana and performed
morphological analysis on Hiragana sentences using a corpus consisting only of Hiragana.
Moriyama et al. (2018) [8] developed morphological analyzer for Kana sentences using
Recurrent Neural Network Language Model. We also developed a morphological analyzer
for only Hiragana sentence, but we try to achieve this goal without any dictionary.
There are some studies focus on morphological analyzers for Hiragana-highly-mixed
sentences and most of them treated a lot of Hiragana words as noises or broken Japanese.
For example, Kudo et al. (2012) [4] used generative model to model the process ofgenerating Hiragana noise-mixed sentence. They proposed using a large-scale web
corpus and EM algorithm to estimate the modelʼ s parameters to improve the analysis ofHiragana noise-mixed sentences. Osaki et al. (2016) [9] constructed a corpus for
broken Japanese morphological analysis. They defined new parts of speech and used themfor broken expressions. Fujita et al. (2014) [1] proposed an unsupervised domain
adaptation technique that uses the existing dictionaries and labeled data to build amorphological analyzer by automatically transforming them for the features of the target
domain. Hayashi and Yamamura (2017) [2] reported that adding Hiragana words to thedictionary can improve the accuracy of morphological analysis.
We used Bi-LSTM model to develop a morphological analyzer. Ma et al. (2018) [5]developed a word segmentation model for Chinese using the Bi-LSTM model. They
reported that word segmentation accuracy achieved better accuracy on public datasets
than the Bi-LSTM model, compared to models based on more complex neural networkarchitectures.
Also, Thattianaphanich and Prom-on (2019) [10] developed the Bi-LSMT CRF modeland performed named entity recognition extraction in Thai. In Thai, there are linguistic
problems such as lack of linguistic resources and boundary indices between words,
phrases, and sentences. Therefore, they prepared word representations and learned text
sequences using Bi-LSTM and CRF to address these problems.

3. METHODSIn this research, we used the Bi-LSTM CRF model to generate a morphological analyzer
for Hiragana sentences. We trained and compared the following five models for Hiragana
sentences in our studies.
1.

the Hiragana Wiki model2.

the Hiragana Yahoo! model3.

the Kanji-Kana Wiki+ Hiragana Wiki model4.

the Kanji-Kana Wiki+ Hiragana Wiki+ Hiragana Yahoo! model5.

the Hiragana Wiki+ Hiragana Yahoo! model

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The generation processes of the five models for Hiragana sentences are shown in Figure1.

3.1. Hiragana Wiki Model
The Hiragana Wiki model is the model generated in training B (Figure 1). We used thedata from Wikipedia, where Kanji-Kana mixed sentences are converted to Hiragana, fortraining data. Hiragana is a phonogram, and Kanji could be converted into Hiragana
according to its pronunciation. We used MeCab's reading data as a pseudo-correct answer
for the conversion because Wikipedia does not have Hiragana-only data. Here, UniDic
was used as a dictionary of MeCab. [6]By default, MeCab uses IPAdic as its dictionary. However, BCCWJ corpus used as test
data uses UniDic, so we used Unidic as MeCab's dictionary.

3.2. Hiragana Yahoo! Mode
The model generated in training D (Figure 1) is the Hiragana Yahoo! model. We usedreading data from Yahoo! Answers as training data. We compared its accuracy to a
Hiragana Wiki + Hiragana Yahoo! model (training F in Figure 1) and Kanji-Kana Wiki+
Hiragana Wiki+ Hiragana Yahoo! model (training E in Figure 1).

3.3. Kanji-Kana Wiki+ Hiragana Wiki ModelIn this paper, we proposed fine-tuning using a model with Kanji-Kana mixed sentences.
The Kanji-Kana Wiki+ Hiragana Wiki model is generated using the original Wikipedia
data and Hiragana Wikipedia data, which are the actual data converted into only Hiragana.
We fine-tuned Kanji-Kana Wiki model, the model trained by the original Wikipedia data
(training A in Figure 1), with Hiragana Wikipedia data, which is Wikipedia data
automatically converted into Hiragana sentences (training C in Figure 1). Kanji-Kanamixed sentences contain many clues for morphological analysis, such as borderline
between Kanji and Hiragana and information about Kanji. Therefore, it is expected to
improve accuracy. By comparing the Hiragana Wiki model and this model, we can assess
the effectiveness of fine-tuning based on Kanji-Kana mixed Wikipedia data, when we only
have Wikipedia data.

3.4. Kanji-Kana Wiki+ Hiragana Wiki+ Hiragana Yahoo! Model
Training E in Figure 1 generates the Kanji-Kana Wiki+ Hiragana Wiki+ Hiragana Yahoo!
model. To improve accuracy, we used both Wikipedia and Yahoo! Answers as trainingdata.
We fine-tuned the Kanji-Kana Wiki+ Hiragana Wiki model with Hiragana Yahoo!
Answers data for this model.

3.5. Hiragana Wiki+ Hiragana Yahoo! Model

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The Hiragana Wiki+ Hiragana Yahoo! model is the model generated in training F in
Figure 1. As training data, we used Hiragana Wikipedia data and Hiragana Yahoo!
Answers data. We fine-tuned the Hiragana Wiki model with Hiragana Yahoo! Answers.
We can see the effectiveness of the Kanji-Kana Wiki+ Hiragana Wiki model, i.e., howmuch the Kanji-Kana mixed model affects this model, when we have both Wikipedia data
and Yahoo! Answers by comparing the accuracy of this model to Kanji-Kana Wiki+
Hiragana Wiki+ Hiragana Yahoo! model.

Figure 1. Model generation processes of the five models for Hiragana sentences

4. EXPERIMENT
4.1. Network Architecture
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International Journal on Natural Language Computing (IJNLC) Vol.11, No.1, February 2022

We developed Bi-LSTM CRF models using Bi-LSTM CRF module of Pytorch.
Table 1 summarizes the hyperparameters used in this experiment. The dimensions forthe network architecture were determined according to the preliminary experiments.

Table 1. Hyperparameters used in the experiment
EMBEDDING_DIM

5

HIDDEN_DIM

4

Lr

0.01

Weight decay
Optimizer

1e-4
sgd

Epoch number

15

The tag size is 38 for all models and the vocabulary size varies according to the model
because it is the total character type number of the training corpus. The vocabulary sizeof the models that use only Hiragana sentences are 192 and that of the model that uses
Kanji-Kana Wiki data was 2,742.

4.2. Training Data
We used two kinds of training data in this experiment: Japanese Wikipedia data and“Yahoo Answers Data” from BCCWJ of the National Institute for Japanese Language andLinguistics. For this experiment, the data from Wikipedia were extracted fromjawiki-latest-pages-articles.xml.bz2
which is published on the website for this experiment.
We preprocessed the Wikipedia data before training to obtain Hiragana-only sentences.
The preprocessing procedures are as follows. First, we conducted morphological analysis
on the Wikipedia data using MeCab. The output of MeCab has features: word
segmentation, part of speech, part of speech subdivision 1, part of speech subdivision 2,
part of speech subdivision 3, conjugation type, conjugated and basic forms, reading,pronunciation.

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Table 2. MeCab's analysis result for “僕は君と遊ぶ"
僕(boku)は(ha)君(kimi)と(to)遊ぶ(asobu)part of speechnoun

particlenounparticleverb

part-of-speech
subdivision 1
part-of-speech
subdivision 2

pronounpronoungeneralcase
particle
generalnon-boundgeneralbinding
particle*

part-of-speech
subdivision 3

*

*

*

*

*

conjugation type

*

*

*

*

godan_verb_ba_column

conjugated form

*

*

*

*

basic form

basic form

僕
(boku)は(ha)君(kimi)と(to)遊ぶ(asobu)readingボク(boku)

ハ(ha)

キミ(kimi)ト(to)アソブ(asobu)pronunciation

ボク(boku)

ワ(wa)

キミ(kimi)ト(to)アソブ(asobu)*Let us take the example of the sentence, “僕は君と遊ぶ" (boku wa kimi to asobu). Table
2 shows the output result of MeCab when we input this example sentence. This is a KanjiKana mixed sentence, which means “I play with you." If this sentence is expressed entirelyin Hiragana, that would be “ぼくはきみとあそぶ. "We obtained Hiragana data by replacing the surface forms of words with their readings.
Next, we split the Hiragana data into individual characters and assign a part-of-speech
tag to each of them. Here, B-{Part-of-Speech} is assigned to the first Hiragana character,and I-{Part-of-Speech} is assigned to the following Hiragana characters, if the Kanji
consisted of more than two syllables.
By formatting this Hiragana sentence as described above, we can obtain the character
data and tags corresponding to the character data with one-to-one correspondence (Table
3).

Table 3. Example of splitting “ぼくはきみとあそぶ"
ぼ

く

は

き

み

と

あ

そ

ぶ
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(bo)
BNoun

(ku)
INoun

(ha)
BParticle

(ki)
BNoun

(mi)
I-Noun(to)
BParticle(a)B-Verb(so) (bu)
IIVerb Verb

Please note that Japanese Kanji characters often have more than one reading. For example,“君" could be Kimi or Kun in Japanese. Therefore, sometimes this conversion makes someerrors. The number of characters in the training data is 1,183,624.
We used the original Japanese Wikipedia data for the model that uses Kanji-Kana mixed
sentences. Table 4 shows the characters and their tags of the Kanji-Kana mixed sentence,“僕は君と遊ぶ."
Table 4. Example of splitting “僕は君と遊ぶ"
僕(boku)は(ha)君(kimi)

と
(to)

遊ぶ
(aso)

ぶ
(bo)

B-NounBParticle

B-NounB-Particle

B-VerbI-VerbFor Yahoo! Answers data, we extracted the reading data from the corpus.
Native Japanese speakers manually annotate them, but sometimes they could be different
from the authors' intent. For example, “⽇本" (Japan in Japanese) has two readings,“Nihon" and “Nippon," and both of them are correct in most sentences. Therefore, inthese cases, only the author can guarantee which one is correct. In BCCWJ, some words
had a predefined reading to reduce the burden of annotators.

4.3. Test Data
We also used the BCCWJ for the test data.
The BCCWJ provides sub-corpora and we used 12 of them.
The number of characters used in this experiment for each dataset is shown in Table 5.

Table 5. Number of characters for each sub-corpus in BCCWJ
Dataset Name

Number of characters

Books (Library sub-corpus)

1,374,216

Bestsellers

1,093,860

Yahoo! Answers

830,960

Legal Documents

2,316,374

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National Diet Minutes

2,050,400

PR Documents

2,151,126

Textbooks

956,927

Poems

466,878

Reports
Yahoo! Blogs

2,546,307
1,305,660

Books

1,281,251

Newspapers

1,301,728

The test data are tagged Hiragana characters with a one-to-one correspondence between
the Hiragana character and the tag-based on reading and part-of-speech information,
were created in the same way as the training data.
The data from “Yahoo! Answers" are also used as training data for creating the Hiragana
Yahoo! model, the Hiragana Wiki + Hiragana Yahoo! model, and the Kanji-Kana Wiki+
Hiragana Wiki+ Hiragana Yahoo! model, however we used different parts for the training
and testing.

4.4. BCCWJ Data
The BCCWJ data we used is shown in Table 5. Twelve datasets exist, each with a different
genre of data. The following section describes from which media and when each dataset
was collected. [11]1. Books (Library sub-corpus)

Books (Library sub-corpus) is a random sample of books published during the 20 years
from 1986 that are held in public libraries in Tokyo.
2. Bestsellers

Bestsellers are a random sample of books that became bestsellers during a 30-year period
from 1976.
3. Yahoo! Answers

Yahoo! Answers is a randomly selected sample of data posted on the Q&A-styleknowledge community service Yahoo! Answers. The original data of Yahoo! Answers
includes 3,120,839 questions posted between October 2004 and October 2005, andmultiple answers to them.
4. Legal Documents

Legal Documents is a random sample of all laws promulgated during the 30 years from1976 and are in effect in 2009, that is the year the corpus compiled.
5. National Diet Minutes

National Diet Minutes uses meeting minutes data from 32,986 meetings held between
the 77th and 163rd Congresses.
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6. PR Documents

The PR Document is defined as the population of PR Documents published in 2008 by
100 local governments selected from all over Japan. PR Documents has selected 100
municipalities from all over Japan, taking into account the region and population
composition. The population of the 100 municipalities is defined as the number of PR
newspapers published in 2008.
7. Textbooks

Textbooks is randomly sampled from textbooks used by elementary, junior high, and highschool students. However, some specialized subjects in high schools like agriculture andcommerce are excluded.
8. Poems

Poems was sampled from substitute works for three types of poems: Tanka, Haiku, andpoem.
9. Reports

Reports were randomly sampled as Report published between 1976 and 30 years.
10. Yahoo! Blogs

The Yahoo! Blogs is a random sample of article data from the Yahoo! Blogs. The original
data from Yahoo! Blogs contained a total of 3,463,413 articles.

11. Books

Books is a random sample of all books published in Japan during a five-year period
starting in 2001.
12. Newspapers

Newspapers were randomly sampled from all newspapers published in Japan during afive-year period starting in 2001.

5. RESULT
According to genres of the test data, Tables 6 and 7 summarize the accuracies of theHiragana Wiki model, Kanji-Kana Wiki+ Hiragana Wiki model, Kanji-Kana Wiki+
Hiragana Wiki+ Hiragana Yahoo! model, Hiragana Yahoo! model, and Hiragana Wiki+
Hiragana Yahoo! model. For the evaluation of each test dataset, we used macro and microaverages of accuracy. Macro represents the macro-averaged accuracy, and micro
represents the micro-averaged accuracy in Tables 6 and 7. In Table 6, blue numbers
indicate that the accuracy of the Kanji-Kana Wiki+ Hiragana Wiki model or the KanjiKana Wiki+ Hiragana Wiki+ Hiragana Yahoo! model was lower than that of the Hiragana
Wiki model, and an asterisk indicates that this difference was significant according to a38

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chi-square test for accuracy at the 5% significance level. In Table 7, Magenta numbers
indicate that they are lower than the accuracy of the Kanji-Kana Wiki + Hiragana Wiki+
Hiragana Yahoo! model, and italics indicate that they are lower than the accuracy of theHiragana Yahoo! model. An asterisk indicates that the difference was significant
according to a chi-square test for accuracy at the 5% significance level. A plus indicatesthat the Hiragana Wiki+ Hiragana Yahoo! model was different from the Hiragana Yahoo!
model according to a chi-square test for accuracy at the 5% significance level. The bold
numbers are the best results of the models in Tables 6 and 7.
For the Kanji-Kana Wiki+ Hiragana Wiki+ Hiragana Yahoo! model, the Hiragana Yahoo!
model, and the Hiragana Wiki+ Hiragana Yahoo! model, the accuracies of Yahoo!
Answers, are results of the closed test, and that is why they are written in parentheses.
Also, the Yahoo! Answers evaluation data are removed from the macro and micro averagesof all models. Therefore, they are average of 11 sub-corpora except for Yahoo! Answers
data.

Table 6. Accuracy of Hiragana Wiki model, Kanji-Kana Wiki+ Hiragana Wiki model,
and Kanji-Kana Wiki+ Hiragana Wiki+ Hiragana Yahoo! model according to each text
genre. Blue numbers mean they are lower than the accuracy of the Hiragana Wiki modeland an asterisk indicates that the model was different from the Hiragana Wiki modelaccording to a chi-square test at the 5% level of significance. The bold numbers are thebest results of the models.
Hiragana Wiki

Kanji-Kana Wiki+
Hiragana Wiki

Kanji-Kana Wiki+
Hiragana Wiki+
Hiragana Yahoo!

Books (Library
sub-corpus)

56.98*57.15*62.76

Bestsellers
Yahoo! Answers

50.14
48.32
63.39

*50.71*50.23*60.48(*65.50)

63.32*60.55National Diet
Minutes

48.49*49.59*60.63PR Documents

64.27*63.93*64.55

Textbooks

57.43*58.01*62.04

Poems
Reports

41.96
65.83

*42.45*65.57*48.82*65.15

Yahoo! Blogs

57.12

57.10

*62.97

Books
Newspapers

55.73
62.58

55.77
*62.30*62.83*64.50

Macro

56.72

56.90

61.39

Legal Documents

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Micro

58.67

58.80

62.44

Table 7. Accuracy of Kanji-Kana Wiki+ Hiragana Wiki+ Hiragana Yahoo! model,
Hiragana Yahoo! model, and Hiragana Wiki+ Hiragana Yahoo! model according to eachtext genre. Magenta numbers indicate that they are lower than the accuracy of the
Kanji-Kana Wiki + Hiragana Wiki+ Hiragana Yahoo! model, and italics indicate thatthey are lower than the accuracy of the Hiragana Yahoo! model. An asterisk indicatesthat the model was different from the Kanji-Kana Wiki + Hiragana Wiki+ Hiragana
Yahoo! model according to a chi-square test at the 5% significance level. A plus
indicates that the model is different from Hiragana Yahoo! model at the 5% significancelevel. The bold numbers are the best results of the models.
Kanji-Kana Wiki+
Hiragana Wiki+
Hiragana Yahoo!

Hiragana Yahoo!

Hiragana Wiki+
Hiragana Yahoo!

Books (Library
sub-corpus)
Bestsellers

62.76*63.11*+63.52

60.48*60.16*+61.32

Yahoo! Answers(65.50)(*65.85)*+66.38

Legal Documents

60.55*60.17National Diet
Minutes

60.63*60.49*+62.63*+61.97

PR Documents

64.55*65.49*+63.91

Textbooks
Poems

62.04
48.82

*+62.66

Reports

65.15

61.97
*47.61*66.89Yahoo! Blogs

62.97*64.02+62.90Books
Newspapers

62.83
64.50

*63.28*65.54*63.36*+49.55*+64.89+64.5140

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Macro
Micro

61.39
62.44

61.70
62.93

61.93
63.01

We also evaluated the training data using MeCab. For the dictionary of MeCab, we used
ipadic with the conversion of Kanji into Hiragana. The macro-averaged accuracy was
79.71%, and the micro-averaged accuracy was 80.10%. Please note that this result cannot
simply be compared with results in Tables 6 and 7 because our system did not use a
dictionary for the morphological analysis itself.

6. DISCUSSION
Table 6 shows that the Kanji-Kana Wiki+ Hiragana Wiki model's macro and microaveraged accuracies (56.90% and 58.80%) are higher than those of the Hiragana Wikimodel (56.72% and 58.67%). The macro-averaged accuracy of the Kanji-Kana Wiki+
Hiragana Wiki model improved by 0.18 points, while the micro-averaged accuracy by 0.13
points. This result indicates that the fine-tuning using the Kanji-Kana Wiki model issomehow effective.
Furthermore, the macro and micro-averaged accuracies of the Kanji-Kana Wiki+
Hiragana Wiki+ Hiragana Yahoo! model (61.39% and 62.44%) are superior to those of
the Kanji-Kana Wiki+ Hiragana Wiki model (56.90% and 58.80%). The Kanji-KanaWiki+ Hiragana Wiki+ Hiragana Yahoo! model improved the accuracy by 4.49 points on
the macro-averaged accuracy and by 3.64 points on the micro-averaged, indicating thatfurther fine-tuning using Hiragana Yahoo! data considerably improve the permanence.
Additionally, from Tables 6 and 7, we can confirm that the macro- and micro-averaged
accuracies of the Kanji-Kana Wiki+ Hiragana Wiki+ Hiragana Yahoo!, Hiragana Yahoo!,and Hiragana Wiki+ Hiragana Yahoo! models, the models using Yahoo! Answers as
training data, are higher than those of Wikipedia and Kanji-Kana Wiki+ Hiragana Wikimodels, the models using only Wikipedia as training data. In other words, theperformance of the model is better when using Yahoo! Answers. We believe there could
be two reasons for this result, the quality of the corpus and the similarity of the training
and test data. The Hiragana Yahoo! data quality could be better than the Hiragana Wikidata because Hiragana Yahoo! data are manually annotated, whereas Hiragana Wiki data
are automatically generated. Moreover, because the training and test data are both subcorpora of BCCWJ, they can be more similar than when the training and test data are
Wikipedia and BCCWJ.
Additionally, comparing the macro- and micro-averaged accuracies of the Kanji-KanaWiki+ Hiragana Wiki+ Hiragana Yahoo! model with those of the Hiragana Yahoo! modeland the Hiragana Wiki+ Hiragana Yahoo! model in Table 7, we confirmed that theHiragana Wiki+ Hiragana Yahoo! model is the best, and the Hiragana Yahoo! model isthe second best, and the Kanji-Kana Wiki+ Hiragana Wiki+ Hiragana Yahoo! model isthe last. This result indicates that when the Hiragana Yahoo! data are available, the finetuning using both the Kanji-Kana Wiki model and the Hiragana Wiki model is not41

International Journal on Natural Language Computing (IJNLC) Vol.11, No.1, February 2022

effective, although the fine-tuning using only the Hiragana Wiki model is useful. Notably,
the accuracy of some types of test data is improved while others are decreased. The KanjiKana Wiki+ Hiragana Wiki+ Hiragana Yahoo! model was higher than the Hiragana
Yahoo! model for the “Bestsellers," “Legal Documents," “National Diet Minutes,"“Textbook," and “Poems", and it was higher than the Hiragana Wiki+ Hiragana Yahoo!
model for “PR Documents", “Reports", and “Yahoo! Blogs" but there was no genre where
the Kanji-Kana Wiki+ Hiragana Wiki+ Hiragana Yahoo! model was the best in these threemodels. Therefore, we think that the reason why the Kanji-Kana Wiki model was usefulwhen only the Hiragana Wiki data were available could be that the Hiragana Wiki data
were automatically generated.
Now, let us discuss the difference among the genres of the texts. The genres where theaccuracy of the Hiragana Wiki model was more than 60% were four genres: “Legal
Documents," “PR Documents," “Reports," and “Newspapers."
We believe this is because the writing style of these test data is close to that of Wikipedia.
Therefore, we marked these genres with underline in Tables 6 and 7. Additionally, we cansee that, as for these genres, the fine-tuning is not useful. The accuracies of the KanjiKana Wiki+ Hiragana Wiki model decreased for these four genres, and those of the KanjiKana Wiki+ Hiragana Wiki+ Hiragana Yahoo! model also decreased for two genres. As
for the remaining two genres, the improvements are less than two points.
Figure 2 shows the effects or the differences of the accuracies of Hiragana Yahoo! dataaccording to the genres of the texts. The blue line is its effects when the base model is the
Kanji-Kana Wiki+ Hiragana Wiki model and the orange line is when the base model isthe Hiragana Wiki model. This figure shows that the effects are the same even though thebase models are different.

Figure 2. Effect of Hiragana Yahoo! data

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Hiragana Yahoo! data was considerably effective for “Yahoo! Answers," “Books," and“National Diet Minutes." Hiragana Yahoo! data is text data of the question answeringsites. Therefore, the writing style is rather like spoken language. We believe that it iseffective for “Books" and “National Diet Minutes" because the Hiragana Books include
dialogues and National Diet Minutes is transcription of the discussion of the Diet. In
particular, Wikipedia data rarely include question sentences but question answering sites
contain many. Therefore, Hiragana Yahoo! data is effective for “National Diet Minutes"
that includes many questions.
Figure 3 shows the effects or the differences of the accuracies of Kanji-Kana Wiki dataaccording to the genres of the texts. The blue line is its effects when the base model is theHiragana Wiki model, and the orange line is when the base model is the Hiragana Wiki+
Hiragana Yahoo! model. As contrasted to Figure 2, the effects of the data are almost
opposite depending on the base model. The blue line is similar to the lines in Figure 2 but
the orange line is very different from those. The orange line, the effects of the Hiragana
Yahoo! data when it was added to the Hiragana Wiki+ Hiragana Yahoo! model shows thatthe data is effective for “PR Documents,” “Reports," and “Yahoo! Blogs." The Kanji-KanaWiki data tends to effective when the fine-tuning of the Hiragana Yahoo! is not effective.
These facts indicate that the fine-tuning is effective when the original accuracy was poor,
regardless of whether it used Kanji-Kana Wiki text or Hiragana Yahoo! data.

Figure 3. Effect of Kanji-Kana Wiki data

Finally, the accuracy of the Bi-LSTM CRF model still has room for improvementcompared to the existing morphological analyzer for Kanji-Kana mixed text. We thinkthat the use of dictionary and much more data should be attempted in the future.

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International Journal on Natural Language Computing (IJNLC) Vol.11, No.1, February 2022

7. CONCLUSIONSThis study developed a morphological analysis model for Japanese Hiragana sentencesusing the Bi-LSTM CRF model. We showed that the performance of morphologicalanalysis of Hiragana sentences outperformed when we use in-domain data manuallyannotated for fine-tuning by the experiments using Hiragana data of Wikipedia andYahoo! Answers data. We also showed that the performance of Hiragana morphologicalanalysis is improved when we fine-tune the model of Hiragana sentences with Kanji-Kanamixed sentences. This fine-tuning is effective when the original accuracy was low.
Additionally, we showed that the performance of morphological analysis varied according
to the text genre. In the future, we would like to increase the number of training data to
improve the accuracy of the model for analyzing Hiragana sentences. Additionally, we
would like to make the system capable of outputting word reading, pronunciation, andpart-of-speech classification. The current system is not capable of outputting thisinformation. Since everything is composed of hiragana, there is a very high possibility thatword sense ambiguity will occur. Therefore, it will be important to solve this problem inorder to continue the research.
In this experiment, we used the Yahoo! Answers as a training example for the Kanji-KanaWiki+ Hiragana Wiki+ Hiragana Yahoo! model and Hiragana Yahoo! models to measure
the accuracy, but we would like to train the models using other data to analyze the effectof genre differences. Furthermore, we would like to investigate the effective domain
adaptation method when the original accuracy is not bad.

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