MORPHOLOGICAL SEGMENTATION WITH LSTM NEURAL NETWORKS FOR TIGRINYA

International Journal on Natural Language Computing (IJNLC) Vol. 7, No.2, April 2018
DOI: 10.5121/ijnlc.2018.7203 29
MORPHOLOGICAL SEGMENTATION WITH LSTM
NEURAL NETWORKS FOR TIGRINYA
Yemane Tedla and Kazuhide Yamamoto
Natural Language Processing Lab, Nagaoka University of Technology,
Nagaoka City, Japan
ABSTRACT
Morphological segmentation is a fundamental task in language processing. Some languages, such as
Arabic and Tigrinya,have words packed with very rich morphological information.Therefore, unpacking
this information becomes a necessary taskfor many downstream natural language processing tasks. This
paper presents the first morphological segmentation research forTigrinya. We constructed a new
morphologically segmented corpus with 45,127 manually segmented tokens. Conditional random fields
(CRF) and window-based longshort-term memory (LSTM) neural networkswere employed separately to
develop our boundary detection models. We appliedlanguage-independent character and substring features
for the CRFand character embeddings for the LSTM networks. Experimentswere performed with four
variants of the Begin-Inside-Outside (BIO) chunk annotation scheme. We achieved 94.67% F1 scoreusing
bidirectional LSTMs with fixed-sizewindow approach to morphemeboundary detection.
KEYWORDS
Morphological segmentation, Tigrinyalanguage,LSTM neural networks, CRF
1. INTRODUCTION
Morphological processing at the word level is usually the initial step in the different stages of
natural language processing (NLP). Morphemes constitute the minimal meaning-bearing units in
a language [1]. In this paper, we focus on the task of detecting morphological boundaries, which
is alsoreferredto as morphological segmentation. This task involves the breaking down of words
into their component morphemes. For example, the English word “reads”can be segmented into
“read”and “s”, where “read”is the stem and “-s”is an inflectional morpheme, marking third person
singular verb.
Morphological segmentation is useful for several downstream NLP tasks, such as morphological
analysis, POS tagging, stemming, and lemmatization. Segmentation is also applied as an
important preprocessing phase in a number of systems including machine translation, information
retrieval, and speech recognition. Segmentation is mainly performed using rule-based approaches
or machine learning approaches. Rule-based approaches can be quite expensive and language-
dependent because the morphemes and all the affixation rules need to be identified to
disambiguate segmentation boundaries. Machine learning approach, on the other hand, is data-
driven wherein the underlying structure is automatically extracted from the data. In this paper, we
present supervised morphological segmentation based on CRFs [2] and LSTM neural networks
[3]. Since morphemes are sequences of characters, we address the problem as a sequence tagging
task and propose a fixed-size window approach for modeling contextual information of
characters. CRFs are well-suited for this kind of sequence aware classification tasks. We also
exploit the long-distance memory capabilities of LSTMs for modeling boundaries of morphemes.
Our main contributions are the following.
1. We constructed the first morphologically segmented corpus for Tigrinya. This corpus is
annotated with boundaries that identify prefix, stem, and suffix morphemes.

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2. We present the first supervised morphological segmentation research for Tigrinya.
Extensive experiments were performed for different annotation schemes and learning
approaches exploiting character embeddings as the sole features for LSTMs. We also
compare our results with feature-rich CRF-based segmentation employing language-
agnostic character and substring features.
3. Tigrinya is an understudied language. Therefore, through this fundamental research, we
hope to contribute in bettering the understanding of the properties and language processing
challenges of the language and encourage further research.
This paper is organized as follows. In section 2, the Tigrinya morphology will be briefly
introduced. In section 3, relevant previous works will be discussed. Section 4 describes the CRF
and LSTM-based methods employed in this research. In the sections that follow, the experimental
settings will be explained and the results discussed. Finally, concluding remarks will be provided.
2. TIGRINYA LANGUAGE
2.1. WRITING SYSTEM
Tigrinya is one of the few African languages that still use an indigenous writing system for
education and daily communication. The writing system, known as the Ge’ez script, is adopted
from the ancient Ge’ez language, which is currently used as a liturgical language. The Ge’ez
script is an abugida system in which each letter (alphabet) represents a consonant-vowel (CV)
syllable. The Tigrinya alphabet chart, known as “Fidel”, comprises of about 275symbols.
Gemination is not explicit in the Ge’ez script; however, this limitation does not seem to pose a
problem for native speakers.In this paper, Tigrinya words are transliterated to Latin characters
according to the SERA scheme with the addition of “I”for the explicit marking of the epenthetic
vowel known as “SadIsI”.The SERA transliteration scheme is available at
ftp://ftp.geez.org/pub/sera-docs/sera-faq.txt. We directly apply labeling to the Latin
transliterations of Tigrinya words and not the Ge’ez script. The Ge’ez script is syllabic, and, in
many cases, the boundary has fusional properties resulting in alterations of characters at the
boundary. For example, the word “sebere”(He broke) would be segmented as “seber”+
“e”because the morpheme “e”represents grammatical features. However, this morpheme cannot
be isolated from the Ge’ez script because the last characters “re”forms a single symbol in the
Ge’ez script and segmenting “sebere”as “sebe”+ “re”is not a correct analysis.
2.2. TIGRINYA MORPHOLOGY
Tigrinya is a Semitic language spoken by over 7 million people in Eritrea and Ethiopia. The
morphology of Semitic languages, known as “root-and-pattern”morphology, has distinct non-
concatenative properties that intercalate consonantal roots and vowel patterns [4]. For example, in
Tigrinya, the words “sebere”(he broke) and “sebira”(she broke) share a common tri-consonantal
root or radical “s-b-r”but have different sequences of vowel patterns (“e-e-e”and “e-i-a”) that
are inserted in-between the radicals. In addition to such a unique infixation, words are formed by
affixing morphemes of prefix, suffix, as well as circumfix. These morphemes represent
morphological features including gender, person, number, tense, aspect, mood, voice, and so on.
Furthermore, there are clitics of mostly prepositions and conjunctions that can be affixed in other
words. These components are arranged in the following manner;
(proclitics)(prefix/circumfix)(root-with-infix)(circumfix/suffixes)(enclitics)
Specifically, the order of morpheme slots is defined by [5] as follows.
(prep|conj)(rel)(neg)sbjSTEMsbj(obj)(neg)(conj)

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The slots “prep”and “conj”are affixes of prepositions or conjunctions attached before or after the
word. The “rel”indicates a relativizer (the prefix “zI”) corresponding in function to the English
demonstratives like that, which, and who. The “sbj”on either side ofthe STEM are a prefix and/or
suffix of the four verb types namely; perfective, imperfective, imperative, and gerundive. As
shown in examples 1-4, the perfective and gerundive verbs conjugate only on suffixes (examples
1 and 4) while imperfective verbs undergo both the prefix and suffix inflections (example 2). The
imperatives show the suffix only conjugations or change prefix as well (example 3). In addition to
the verb type, these fusional morphemes convey gender, person, and number information.
1. seber + u (they broke)
2. yI + sebIr + u (they break)
3. yI + sIber + u (break/let them break)
4.sebir + omI(they broke)
Moreover, Tigrinya independent pronouns have a pronominal suffix (SUF) of gender, person, and
number as shown in examples 5 and 6.
5.nIsu (he) →nIs + u/SUF
6. nIsa(she) →nIs + a/SUF
The word order typology is normally subject-object-verb (SOV), though there are cases in which
this sequence may not apply strictly [6, 7]. Changes in “sbj”verb affixes, along with pronoun
inflections, enforce subject-verb agreements. One aspect of the non-concatenative morphology in
Tigrinya is the circumfixing of negation morpheme in the structure “ayI-STEM-nI”[5]. In
addition to the slots defined by [5], some enclitics such as question particles“do; ke;Ke”can also
be found in Tigrinya orthography as free or bound suffix morphemes. For example,
KeyIdudo?→keyId + u + do (did he go?)
nIsuKe? →nIs + u + Ke (what about him?)
The pronominal object marker “obj”is always suffixed to the verbas shown in examples 7-10.
According to [6], Tigrinyasuffixes of object pronoun can be categorized into two constructs.The
first is described in relation to verbs (examples 7, 8 and 9) and another indicates the semantic role
of applicative cases by inflecting for prepositional enclitic “lI”+ a pronominal suffix as in
example 10.
7. beliOI + wo/obj (he ate [something])
8. hibu + wo /obj (he gave [something] to him)
9. hibu + ni /obj(he gave me [something])
10. beliOI + lu /obj (he ate for him/he ate using [it])
Tigrinya words are also generated by derivational morphology. There are up to eight derivational
categories that can originate from a single verb [8]. For example, the passive form of perfective
(example11) and gerundive verbs (example 12) is constructed by prefixing “te”to the main verb.
Furthermore, adverbs can be derived from nouns by prefixing “bI-”which has similar
functionality as the English “-ly”suffix (example 13).
11. zekere (he remembered) →te + zekere (he was remembered)
12. zekiru(he remembered) →te + zekiru (he was remembered)
13. bIHaqi(truly)→bI + Haqi
In this work, we deal with boundaries of prefix, stem, and suffix morphemes. Inflections related
to infixes (internal stem changes) are not feasible for this type of segmentation.

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In conclusion, all the morphological and derivational processes generate a large number of
complex word forms. According to [8], Tigrinya verb inflections, derivations, and combinations
of the slot order can produce more than 100,000 word forms from a single verb root. The
ambiguity and difficulty in relation to segmentation are briefly discussed in the following section.
2.3. MORPHOLOGICAL AMBIGUITY
In segmentation, ambiguity may occur at the word-level or due to sentential context. In Tigrinya,
a major source of ambiguity is when certain character sequences of morphemes are natively
present as part of words. In this case, the characters do not represent grammatical features and
hence segmentation should not be applied.Consider the words “bIrIhanI”(light) and
“bIHayli”(by force). The same prefix “bI-” which is an inseparable part of the noun “bIrIhanI”,
represents an adverb of manner (by) in the second word. Moreover, morphemes may also appear
as constituents of other morphemes. For instance, the noun suffix “-netI”(example 16) contains
the sub-morph “-etI”that can have the role of a suffix for conjugation of third person, feminine,
singular attributes as in example15. Note that “-netI”in example 14 is not a morpheme.
14. genetI→genetI/NOUN_paradise
15. wesenetI→wesen/STEM_decided + etI/SUF_she
16. naxInetI→naxI/STEM_independence + netI/NOUN-SUF
Moreover, the lack of gemination marking may introduce segmentation ambiguity. For example,
the word “medere”can be interpreted as the noun “speech”or the phrase “he gave a speech”if the
“de”in “medere”is geminated. A computational system must discern both cases such that the
phrase is segmented while the noun is left intact. Resolving such cases may require more context
or additional linguistic information such as part-of-speech. In this work, we would like to avoid
resorting to any language-specific knowledge. Therefore, in the LSTM approach, we use
character embeddings to capture contextual dependencies of characters.
As explained earlier, Tigrinya words have multiple consecutive morpheme slots. This pattern
causes under-segmentation confusion due to the numerous intermediate splits comprising atomic
and composite morphemes. Table 1 lists some of the possible segmentations for the word token
“InItezeyIHatetIkayomI”(if you did not ask them).
Furthermore, in Tigrinya, compound words are often written attached. For example, “betI(house)
megIbi(food)”collectively translates to “restaurant”in English. In the orthography, these words
can be found either separate or attached. We queried for “betI-” starting words in a text corpus
containing about 5 million words extracted from the HaddasErtra newspaper,which is published
in Eritrea. The previous word, for instance, occurs delimited by a space in about 95% of the
response. Another compound word “betISIHIfetI” (office) was found attached in about 24% of
the response, which is not a small portion. Although the words arenot grammatical morphemes,
we believe segmenting (normalizing) these words would be useful for practical reasons such as
mitigating data sparseness.
Table 1. Examples of intermediate splits caused by under-segmentation. The italicized second row is the
expected segmentation.
InItezeyIHatetIkayomI
InIte-zeyI-HatetI-ka-yomI
InIte-zeyIHatetIkayomI
InIte-zeyI-HatetIkayomI
InIte-zeyI-HatetI-kayomI
InItezeyIHatetIka-yomI
InItezeyIHatetI-kayomI
InItezeyI-HatetIkayomI

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3. RELATED WORKS
The work of [1] introduced the unsupervised discovery of morphemes based on minimum
description length (MDL) and likelihood optimization. This method lacks the handling of
representing contextual dependencies, such as stem and affix orders. Although several other
unsupervised segmentation approaches have been proposed, it was shown by [9] that minimally
supervised approaches provided better performance compared to solely unsupervised methods
applied on large unlabeled datasets. For example, unsupervised experiments for Estonian, that
achieved 73.3% F1 score with 3.9 million words, was outperformed by a supervised CRF that
attained 82.1% with just 1000 word forms. This work also showed that semi-supervised
approaches that use both annotated and unannotated data can be leveraged to improve upon
simply using completely unsupervised methods. In related works for Semitic languages of
Hebrew and Arabic, [10] uses a probabilistic model where segmentation and morpheme
alignmentsareinferred from the shared structure between both languages using parallel corpus
with and without annotation. Recent works on neural models rely on the use of some form of
embedding for extracting relevant features. [11] demonstrated a generic window-based neural
architecture that is applied to several NLP tasks while avoiding explicit use of feature
engineering. Their system trains on large unlabeled data to learn internal representations. We
have also adopted a similar window-based approach although with the input of character
sequence window instead of words as in [11]. In [12], state-of-the-art results were achieved with a
neural model that learns POS related features only from character-level and sub-word
embeddings. Other research, however, argue that enriching embeddings with additional
morphological information boost performance. For example, [13] demonstrates this by using the
results of a morphological analyzer to further improve candidate ranking in a morphological
disambiguation task for Arabic. In a research for Burmese word segmentation, [14] address the
problem by employing binary classification with classifiers such as CRFs. The tagset restriction
to the binary was mainly due to data size. Our data is similarly small size corpus;however, we
report experiments with several schemes to investigate the effect of using simple to more
expressive tags in the Tigrinya morpheme segmentation.
Amharic and Tigrinya are closely related languages. These languages share a number of
grammatical features and vocabularies. [16]presentedmorphological rule learning and
segmentation based on inductive logic programming where rules or affixes were learned by
exposing easy-to-complex examples incrementally to an intelligent teacher. Their system for affix
segmentation achieved a performance of 0.94 precision and 0.97 recall measures. Tigrinya
remains an under-studied and under-resourced language from the NLP perspective. However, as
regards to morphological processing, [8] employed finite state transducers (FSTs) to develop,
“HornMorpho”, a morphological analysis and generation system for Tigrinya, Amharic, and
Oromo languages. The FST empowered by feature structures was effectively adopted to process
the unusual non-concatenative root-and-pattern morphology. The Tigrinya module of
HornMorph2.5 performs the full analysis of Tigrinya verb classes [5]. The system was tested on
randomly selected 400 word forms with 329 of them being non-ambiguous. The analysis
revealedremarkably accurate results with a few errors due to unhandled FSTs, which can be
integrated. Our approach is different from HornMorph in at least two aspects. First, our task is
limited to identifying morphological boundaries. The results amount to partially analyzed
segments although these segments are not explicitly annotated for grammatical feature. The
annotation could be pursued with further processing of the output or training on morphologically
annotated data, which is currently missing for Tigrinya. Second, the use of the FSTs relies heavily
on the linguistic knowledge of the language in question. This would require time-consuming
manual construction of language-specific rules, which is more challenging for root-and-pattern
morphology. In contrast, we follow a data-driven (machine learning) approach to automatically
extract features of the language from a relatively small boundary annotated data. Moreover, on
the limitations of HornMorph, [5] notedthat analysis incurs a “considerable time”to exhaust all

34
options before the system responds. Besides HornMorph, there was an attempt to use affix based
shallow segmentation in the pre-processing phase for improving word alignment in the English-
Tigrinyastatistical machine translation [16].
4. METHOD
4.1. MORPHOLOGICALLY SEGMENTED CORPUS
There is no publicly available morphologically segmented resource for Tigrinya. Therefore, we
based our studies on a new morphologically segmented corpus developed in-house. The first
version of this corpus comprisesover 45,000 tokens derived from randomly selected 2774
sentencesofthe part-of-speech tagged Nagaoka Tigrinya Corpus (NTC) available
athttp://eng.jnlp.org/yemane/ntigcorpus.
For the purpose of boundary detection, we employed character-based BIO chunking scheme,
which allows us to exploit character dependencies and alleviate out-of-vocabulary (OOV)
problems by reducing the morpheme vocabulary to about 60 Latin characters that cover the
transliteration mapping we adopted.
4.2. TAGGING SCHEMES
The popular IOB tagging scheme is used to annotate the Beginning (B), Inside (I), and outside
(O) of chunks in tasks such as base phrase chunking and named entity recognition. Similarly, we
address morpheme boundary detection by annotating every character in morpheme chunks with
the appropriate IOB label. There are different variants of the IOB scheme including BIO, IOB,
BIE, IOBES, and so on. There is no general consensus as to which variant performs best. [11]
usedthe IOBES format as it encodedmore information whereas extensive evaluations by [17]
showed that the BIO has superior results compared to the IOB scheme. Furthermore, the BIES
scheme gave better results for Japanese word segmentation [18].
Table 2.Annotating with differenttagging schemes
Scheme
tImaliInItezeyIHatete
(if he did not ask yesterday)    
Word t I m a l i I n I t e z e y I H a t e t e
BIE B I I I I E B I I I E B I I E B I I I I E
BIES B I I I I E B I I I E B I I E B I I I I S
BIO O O O O O O B I I I I B I I I B I I I I I
BIOES O O O O O O B I I I E B I I E B I I I E S
We experimented with four schemes, namely, BIE, BIES, BIO, and BIOES to explore the
modeling of morpheme segmentation in a low-resource setting with morphologically rich
language Tigrinya. In our case, the B, I, and E tags marks Begin, Inside, and End of multi-
character morphemes. The S tag annotates single character morpheme and the O tag assigns
character sequences outside of morpheme chunks. An example of using these annotations is
presented in Table 2.
4.3.CHARACTER EMBEDDINGS
Morphological segmentation is primarily character-level analysis. For example, in Tigrinya, the
characters “zI”have a grammatical role when used as a relativizer that is prefixed only to the
perfective (example 17) or imperfective verbs (example 18). Therefore, all other occurrences of
“zI”, such as the noun “zInabI”(rain) should not be segmented. Consequently, recognizing the
shape of relativized perfectives and imperfectives becomes crucial.

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17. zIsebere‘that broke’ →zI + sebere/PERFECTIVE
18. zIsebIrI‘that breaks’ →→→→zI + sebIrI/IMPERFECTIVE
19. *zI + yIsIberI/IMPERATIVE – invalid
20. zInabI‘rain’ →→→→*zI + nabI- incorrect segmentation
Character-level information must be extracted to learn such informative features useful for
identifying boundaries. We generated past and future fixed-width character context for each
character. The concatenated contextual characters form a single feature vector for the central
target character. For example, the features of the word “selamI”(peace, hello) for each character
are generated as depicted in Table 3. We also showed the corresponding label of the central
character in the BIE scheme. The Boldface character represents the central character with its left
(past) and right (future) context of width five characters. Characters are padded with underscores
(_) to complete the remaining slots depending on the window size.
Table 3. An example of generated fixed window character sequences with assigned label
Window Label
_ _ _ _ _ s e l a m I B
_ _ _ _ s e l a m I _ I
_ _ _ s e l a m I _ _ I
_ _ s e l a m I _ _ _ I
_ s e l a m I _ _ _ _ I
s e l a m I _ _ _ _ _ E
We settledfor a window size of five as increasing it beyond five did not result in significant
improvements. Moreover, the dimension of optimal character embedding was decided by hyper-
parameter tuning experiments. We initialized the embedding layer from a lookup table for integer
(index) representations of the fixed-width character vectors. The embedding is then fed to an
LSTM after passing through a dropout layer.
4.4. CRF
CRFs are probabilistic approaches capable of modeling context-dependent sequence labeling [2].
In a morphological segmentation, the model is trained to predict a sequence of output tags (IOB
labels) = , , … , from a sequence of feature characters = , , … , . The training
task is to maximize the log probabilitylog ( ( | )) of the valid label sequence. The conditional
probability is computed as:
( | ) =
( , )
∑ ( , ) (1)
The equation for the scoring function score is given as:
( , ) = , + ! ,
"
#
"
#$
(2)
%&',&' (
denotes the emission probability of the state change from label ) to label *while +,,- is the
transition probability denoting the score of the*./
label of the)./
word.

36
4.5. LSTM NEURAL NETWORK
Recurrent Neural Networks (RNNs) are feed forward neural networks with feedback cycles to
capture time dynamics using back-propagationthrough time. This recurrent connection allows
thenetwork to employ the current inputs as well as previously encountered states. Given the input
vector 0., ( in our case, a window offive characters left and right of the target character), the
hiddenstate ℎ. at each time step 2, can be expressed by equation 4.
0. = 3 .45, .4(54 ), … , ., … , .6(54), .657 (3)
ℎ. = 8(9:/ . + 9//ℎ.4 + ;/) (4)
where the 9terms represent the weight matrices, ; is a bias vector and 8 is the activation
function. However, due to the gradient vanishing (very small weight changes) or exploding
problems (very large weight changes), long distance dependencies are not properly propagated in
RNNs. [3] introduced LSTMs to overcome this gradient updating problem. The neurons of
LSTMs called memory blocks are composed of three gates (forget, input and output) that control
the flow of information and a memory cell with self-recurrent connection. The formulae of these
four components are given in equations 5 to 9.In these equations, the input, forget, output and cell
activation vectors are denoted by ), <, =and > respectively; 8 and ? represent sigmoid and tanh
functions respectively; ⨀operation is the Hadamard product; 9 stands for the weight matrices
and ; is the bias.
). = 8(9,: . + 9,/ℎ.4 + ;,)
<. = 8A9B: . + 9B/ℎ.4 + ;BC
>5 = ?(9D: . + 9D/ℎ.4 + ;D)
>. = <. ⨀ >.4 + ). ⨀ >5
=. = 8(9E: . + 9E/ℎ.4 + ;E)
ℎ. = =. ⨀ ? (>. )
(5)
(6)
(7)
(8)
(9)
(10)
The LSTM architecture we propose to use is illustrated in Figure 1. Character-level features
(embeddings) generated by the embedding layer are fed to the forward LSTM network. Dropout
layers are adjusted before and after the LSTM layer to regularize the model and prevent
overfitting. The output of the network is then processed by a fully connected (Dense) layer and
finally, tag probability distributions over all candidates are computed via the softmax classifier.
Figure1: The LSTM network

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Due to the capacity to work well with long-distance dependencies, LSTMs have achieved state-
of-the-art performance in window-based approaches as well as sequence labeling tasks including
morphological segmentation [19], part-of-speech [20] and named entity recognition [21].
Figure2: The BiLSTM neural network
4.6. BIDIRECTIONAL LSTM
LSTMs encode the input representation in the forward pass. In Bidirectional LSTM network
(BiLSTM),the input vector is processed in both forward and backward passesand presented
separately to hidden states. The mechanism is usefulin encoding past (forward) and future
(backward) information ofthe input vector. We illustrate the architecture of the BiLSTM
wepropose to use in Figure 2. Both the forward and backward layeroutputs are calculated by
using the standard LSTM updating equations5 to 9. Accordingly, for every time step 2 the
forward andbackward networks take the layer input 0. and then outputℎ.. The celltakes the input
state >5and the cell output >.as well as the previous cell output =.for training and updating
parameters.Like in LSTMs, the output of each network in BiLSTM canbe expressed as:
. = 8 A9/&ℎ. + ;&C (11)
where9 is the weight matrix, ; is the bias vector of the output layer and 8 is the activation
function of the output layer. Boththe forward and backward networks operate on the input state of
0.and generate the forward outputℎ.
FFFG and the backward outputℎ.
HFFF. Thenthe final output, . from
both networks is combined using operationssuch as multiplication, summation or simple
concatenation asgiven in equation 12. In our case, 8 is a concatenating function.
. = 8(ℎ.
FFFG, ℎ.
HFFF) (12)
This context-dependent representation from both networks ispassed along to the fully-connected
hidden layer (Dense) whichthen propagates it to the output layer.
Embedding
Backward LSTM
Forward LSTM
||
Dense
Softmax
_ _ s e l a m I _ _ _
B | I | O

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5. EXPERIMENTS
In this section, the used data and the experimental settings are reported. Initially, we set training
epochs to hundred and configured early stopping if training continuedwithout loss improvement
for 15 consecutive epochs. We usedKeras (https://www.keras.io/) to develop and
Hyperas(https://github.com/maxpumperla/hyperas) to tune our deep neural networks. Hyperas, in
turn,applieshyperopt for optimization. The algorithm used is Tree-structured Parzen Estimator
approach (TPE) over five evaluation trails (https://papers.nips.cc/paper/4443-algorithms-for-
hyperparameter-optimization.pdf).
5.1. SETTINGS
Datasets. The text used in this research is extracted from the NTC corpus. The NTC consists of
around 72,000 POS tagged tokens collected from newspaper articles. Our corpus contains 45,127
tokens, of which 13,336 tokens are unique words. Training is performed with ten-fold cross-
validation, where about 10% of the data in every training iteration is used for development. We
further split the development data into two equal halves allotting one set for validation during
training and the other half for testing the model’s skill.
Table 4. Training data splits as per the count of the fixed-width character windows (vectors) used as the
actual input sequences
Data size
All Training Validation Test
100% 90% 5% 5%
289,158 260,242 14,458 14,458
The evaluation reports in this paper are based on the final test set results. For every target
character in a word, a fixed-width character window is generated as shown in Table 3. The data
statistics according to the generated character windows is presented in Table 4.
Hyper-parameters.
CRF: We performed preliminary experiments for deciding the regularization algorithm (L1 or
L2) andthe C-parameter values of [0.5, 1, 1.5]. Consequently, L1 regularization and C value of
1.5 were selected as these values gave better results. The character and substring features we
considered spanned a window size of five. The full list of the character features is given as
follows.
a. Left context: -5 to 0, -4 to 0, -3 to 0, -2 to 0, -1 to 0
b. Right context: 0 to 5, 0 to 4, 0 to 3, 0 to 2, 0 to 1
c. Left+Right context: a + b
d. N-grams: bi-grams
LSTM: In order to search for the parameters that yield optimal performance, we explored hyper
parameters that include embedding size, batch size, dropouts, hidden layer neurons, and
optimizers. The complete list of the selected parameters for LSTM is summarized in Table 5. We
achieved similar results for the BIE and BIES tunings as shown in the table. However, the BIO
and BIOES schemes showeddifference in the tuning results and, therefore, these were used
separately.
Embeddings: We tested several embedding dimensions from the set {50, 60, 100, 150, 200, 250}.
Separate runs were made for all types of tag sets.

39
Table 5. LSTM Hyperparameters selected by tuning
Parameter BIE/BIES BIO BIOES
Window 5 5 5
Character embedding 150 150 250
hidden layer size 32 128 128
Batch size 32 256 256
optimizer adam adam RMSProp
Dropouts: Randomly selecting and dropping-out nodes have proven effective at mitigating
overfitting and regularizing the model [22]. We applied dropout on the character embedding as
well as on the inputs and outputs of the LSTM/BiLSTM layer. For example, the selected dropouts
for the BIO-based tunings are 0.07 and 0.5 for the embedding and LSTM output layers
respectively. The dropout probabilities are selected from a uniform distribution over the interval
[0, 0.6].
Batch size: We ran tuning for the batch size of the set {16, 32, 64, 128, 256}.
Hidden layer size:The hidden layer’s size is searched from the set {64, 128, 256}.
Optimizers and learning rate: We investigated the stochastic gradient descent(SGD) with
stepwise learning rate and other more sophisticated algorithms such as AdaDelta[23], Adam [24],
and RMSProp[25]. The SGD learning rate was initialized to 0.1, a momentum of 0.9 with rate
updates for every 10 epochs at a drop rate of 0.5. However, the SGD setting did not result in
significant gains compared to the automatic gradient update methods.
5.2. BASELINE
The baseline considered in this study is the CRF model trained on only character features with a
window size of five. The window size is kept the same with the neural LSTMs to compare the
models subject to the same information. The baseline experiments for the CRF achieved an F1
score of around 60% and 63% using the BIOES and BIO tags respectively. As for the BIE tags,
the performance reachedabout 75.7%. All the subsequent CRF-based experiments employed two
auxiliary features, which were the contextual and n-gram characters listed in section 5.1. As
depicted in Table 6, significant performance enhancements were achieved with the auxiliary
features. However, compared to the LSTM based experiments, the window five based CRF
results were still suboptimal.
5.3. EVALUATION
We report boundary precision(+), boundary recall (I) and boundary F1 scores (J1) as given by
equation 13 to 15. Precision evaluates the percentageof correctly predicted boundaries with
respect to the predictedboundaries and recall measures the percentage of correctlypredicted
boundaries with respect to the true boundaries. F1 scoreis the harmonic mean of precision and
recall and can be interpretedas their weighted average.
+ =
>=LLM>2N LMO)>2MO ;=PQORL)MS
LMO)>2MO ;=PQORL)MS
I =
>=LLM>2N LMO)>2MO ;=PQORL)MS
2LPM ;=PQORL)MS
J1 =
2 ∗ ( LM>)S)=Q ∗ LM>RNN)
LM>)S)=Q + LM>RNN
(13)
(14)
(15)
The correct predictions are the count of true positives while the actual (true) boundaries are these
true positives added with the false negatives. The system proposed predictions can be found from

40
the sum of the true positives and the false positives.The number of I or O classes is found more
than double of theB classes. Therefore, to account for any class imbalance, we computedweighted
macro-averages, in which each class contributionis weighted by the number of samples of that
class.
The macro-averaged precision and recall are calculated by takingthe average of precision and
recall of the individual classes asshown in equations 16 and 17, where Q denotes total number
ofclasses.
VR>L= − RXM. + =
+(>NRSS 1) + +(>NRSS 2) + … + +(>NRSS Q)
Q
VR>L= − RXM. I =
I(>NRSS 1) + I(>NRSS 2) + … + I(>NRSS Q)
Q
(16)
(17)
6. RESULTS
We experimented and compared the performance of three models trained with four different
tagging strategies as explained earlier. The resultsof ten-fold cross-validation are summarized in
Table 6 with P, R,F1 representing precision, recall, and F1 score respectively.
Table 6.Results of CRF, LSTM and BiLSTM experiments with four BIO schemes.
Tagset CRF LSTM BiLSTM
P R F1 P R F1 P R F1
BIE 92.62 92.62 92.62 94.38 94.37 94.37 94.68 94.68 94.67
BIES 92.44 92.44 92.44 94.22 94.20 94.20 94.60 94.59 94.59
BIO 84.88 84.88 84.88 90.91 89.96 89.96 90.16 90.11 90.11
BIOES 83.60 83.60 83.60 88.26 88.21 88.21 88.45 88.39 88.39
Generally, we observed the choice of segmentation schemes affecting the model performance.
Overall, using the BIE scheme resulted in the best performance in all tests. Although the BIOES
tagset is the most expressive, since the corpus is rather small, the simpler tagsets showed better
generalization over the dataset.The BIE-CRF model achieved 92.62% in the F1 score while the
performance of the BIO and BIOES-based CRFsfellto about 84.88% and 83.6% respectively. The
CRF model using the BIO scheme outperformed the CRF model using the BIOES by 1.28%
absolute change. A majority of the errors are associated with under-segmentation and confusion
between I (inside) and O (outside) tags. Masking these differences by dropping the O tag has
proved useful in our experiments. As a result, the CRF model using the BIE scheme outperformed
the BIO-based CRF by around 7.7percentage point. We compared the regular LSTM with its
bidirectional extension. The overall results showed that the BiLSTMs performed slightly better
than the regular LSTMs sharing the same scheme. In the window-based approach, the past and
future context is partly encoded in the feature vector. This window was fed to the regular LSTM
at training time making the information available for both networks quite similar. This may be the
reason for not seeing much variation between the two models. Nevertheless, the additional design
in BiLSTMs to reverse-process the input sequence allows the network to learn more detailed
structures. Therefore, we sawslightly improved results with the BiLSTM network over the regular
LSTM. As with CRFs, a significant increase in the F1 score was achieved when changing from
the BIOES to the BIE scheme. In both LSTM models, we observed again in performance of over
6 percentage point. Overall, in these low-resource experiments, the model generalizedbetter when
the data was tagged with the BIE scheme as this reducedthe model complexity introduced by the
O tags. The BiLSTM model using the BIE scheme performed superior to all others scoring
94.67% in the F1. This result was achieved by employing only character embeddings to extract
morphological features. On the other hand, the CRF model which useda rich set of character + bi-

41
gram + substring features scored around 92.62%. For a fair comparison, we used features of the
CRF model that spanned the same window of characters as the LSTM models. In other words, we
avoided heavy linguistic engineering on the CRF side. Although the features were not
linguistically motivated, achieving an optimal result this way still requires many trials and better
design of features. It is to be noted that additional hand-crafted features and wider windows for
the CRF model may produce better results than what has beenreported. However, from the LSTM
results, we sawthat forgoing feature engineering and extracting features using character
embeddings couldsufficiently encode the morphological information desired for high-
performance boundary detection.
While the results may not be directly comparable due to the difference in settings and evaluation,
two related previous works of[26] and [19] on the Semitic languages of Arabic and Hebrew
arereferred for comparison. We adopted similar LSTM-based fixed-window approach presented
in [19] that investigated Arabic andHebrew morphological segmentation with the S & B dataset
containing6,192 word types [27]. The BiLSTM-based best results reportedare an F1 score of
95.2% on Hebrew and 95.5% on Arabic. [19] employedmore advanced multi-window features
and an ensemble of 10modelsbecause the performance of character window features was not
satisfactory for the small data size used. In comparison,we achieved similar results (see Table 6)
with no further featureengineering other than simple character window embeddings,however, the
dataset available to us is larger with 13, 336 wordtypes.
Prior to the work by [19], the state-of-the-art has been a CRF-based morpheme segmentation
relying on hand-crafted feature setsthat include all left and right substrings of a target character
andan additional bias component [26]. Training on the same S and B dataset, [26] achieved F1
score of 94.5% and 97.8% on Hebrewand Arabic respectively. We notice that the LSTM-based
workby [19] on Arabic dataset, does not outperform the CRF-based method of [26]. We achieved
around 92.62% F1 score with CRF-basedTigrinya experiment that was trained on the feature set
listedin section 5.1 with constraints to span only five left and right charactersmatching the
information exposed to our LSTM settings.
6.1. ERROR ANALYSIS
The models using the BIE/BIES or the BIO/BIOES tagsets achieved comparable results. We
analyzed the effect of the tagset choice by comparingthe BiLSTM models of the BIE and BIO
tagging schemes. For easiercomparison, the confusion matrices (in percentage) of both
experiments are presented jointly in Table 7. The values are paired in X/Yformat where X is a
value from the set {B, I, E}in the BIE schemeand Y is one of {B, I, O}in the BIO scheme. As
mentioned earlier, the number of Ior O classes is about two-fold greater than the B class.
Therefore, there is more confusion stemming from those tags. The B tag represents the morpheme
segmentation boundaries; therefore, the under-segmentation and over-segmentation errors can be
explained in relation to the B tags. Looking at the BIE figures, around 91% of thepredictions for
the B tags are correct. In other words, 9% of the true Btags were confused for the I and O tags,
which together amounts to theunder-segmentation errors. These types of errors are almost
doubledwith the use of the BIO scheme. On the other hand, the over-segmentationerrors are about
5.9% and 7.4% in the BIE and BIO scheme respectively. Over-segmentation occurs when true I,O,
or E tags are confused with the B tag. In this case, morpheme boundaries are predicted while the
characters actually belong to the inside, outside, or end of the morpheme. These results show that
the under-segmentation errors contribute more compared to the over-segmentation errors.
In the BIE scheme results, we observe that the I and B tags are correctly predicted with relatively
higher accuracies than with the BIO scheme. Specifically, the prediction accuracy of the B tag
(boundaries) increased from 82.6% to 91.2% (8.6% absolute change). The main reason is the
considerable reduction of errors introduced due to the confusions of tag I for O or vice-versa in
the BIO scheme. This comparison suggests that improved results can be achieved by denoising

42
the data from the O tag. Therefore, for a low-resource scenario of boundary detection, starting
with the BIE tags may be a better choice for improving model generalization compared to more
complex schemes that are feasible for larger datasets.
Table 7.BiLSTM Confusion matrix comparison for BIE (boldfaced) and BIO schemes in %. Rows and
columns represent predicted and true values respectively.
Predicted
B/B I/I E/O
True
B/B 91.20/82.60 4.54/9.94 4.26/7.46
I/I 2.83/3.21 93.84/90.74 3.34/6.05
E/O 3.09/4.19 9.65/10.83 87.26/84.98
The following sample sentence extracted from the Bible is segmented with all four models. Note
that the training corpus does not include text from the Bible.
Tigrinya: “amIlaKI kea mIrIayuzEbIhIgImIbIlaOuzITIOumIkWluomIabImIdIriabIqWele”
English: “And out of the earth the Lord made every tree to come, delighting the eye and good for
food”. 
The expected segmentation is given under the column labeled “True”. We observe that the BIE
model segmentation is the nearest to the reference segmentation. The other models showed under-
segmentation errors for the word “ZEbIhIgI”and “kWlu”. The verbal noun prefix “mI-”is
correctly segmented by all models whereas the models failed to recognize the boundary of the
causative prefix “a-”in the last word “abIqWele”.
Table 8. Sample of segmentation result; the underscore character marks segmentation boundaries and the
boldfaced characters denote segmentation errors.
Sentence True BIE BIES BIO BIEOS
amIlaKI amIlaKI amIlaKI amIlaKI amIlaKI amIlaKI
Kea kea kea kea kea Kea
mIrIayu mI_rIay_u mI_rIay_u mI_rIay_u mI_rIay_u mI_rIay_u
zEbIhIgI zE_bIhIgI zE_bIhIgI zEbIhIgI zEbIhIgI zEbIhIgI
mIbIlaOu mI_bIlaO_u mI_bIlaO_u mI_bIlaO_u mI_bIlaO_u mI_bIlaO_u
zITIOumI zI_TIOumI zI_TIOumI zI_TIOumI zI_TIOumI zI_TIOumI
kWlu kWl_u kWl_u kWlu kWlu kWlu
omI omI omI omI omI omI
abI abI abI abI abI abI
mIdIri mIdIri mIdIri mIdIri mIdIri mIdIri
abIqWele a_bIqWel_e abIqWel_e abIqWel_e abIqWel_e abIqWel_e
7. CONCLUSION AND FUTURE WORK
In this work, we presented the first morphological segmentation research for the morphologically
rich Tigrinya language. The research was performed based on a new manually segmented corpus
comprising over 45,000 words. Four variants of the BIO chunk annotation scheme were
employed to train three different morphological segmentation models. The first model wasbased
on CRFs with language-independent features of characters, n-grams, and substrings. The other
two were based on LSTM and BiLSTM deep neural architectures leveraging character
embeddings of a fixed-size window for extracting the morpheme boundary features. We
evaluated the BIE, BIES, BIO, and BIOES tagging schemes for morphological boundary

43
detection. Although the size of the corpus is relatively small, we achieved 94.67% F1 score in
boundary detection using the BIE chunking scheme.
In the future, we plan to improve and enlarge the corpus by including vocabularies from different
domains. This will allow the inclusion of potentially unseen patterns as the current orthographic
style is limited to news domain. We also plan to extend our experiments to use embeddings with
character and substring concatenated features for the BIO and BIOES schemes, which currently
have lower performance. We are also interested in integrating minimally supervised approaches
to make use of large unlabeled datasets. Segmented words have proved useful in mitigating the
adverse effects of data sparseness in Semitic language processing [28]. We would like to explore
the use of our segmentation output in machine translation, full-fledged morphological analysis,
stemming, part of speech tagging, and other downstream NLP tasks.
ACKNOWLEDGEMENT
We would like to thank Dr. Chenchen Ding, from NICT Japan, for his constructive comments.
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