HINDI NAMED ENTITY RECOGNITION BY AGGREGATING RULE BASED HEURISTICS AND HIDDEN MARKOV MODEL

International Journal of Information Sciences and Techniques (IJIST) Vol.2, No.6, November 2012
DOI : 10.5121/ijist.2012.2604 43
HINDI NAMED ENTITY RECOGNITION BY
AGGREGATING RULE BASED HEURISTICS AND
HIDDEN MARKOV MODEL
Deepti Chopra, Nusrat Jahan, Sudha Morwal
Department of Computer Engineering, Banasthali Vidyapith Jaipur (Raj.), INDIA
deeptichopra11@yahoo.co.in
nusratkota@gmail.com
sudha_morwal@yahoo.co.in
ABSTRACT
Named entity recognition (NER) is one of the applications of Natural Language Processing and is regarded
as the subtask of information retrieval. NER is the process to detect Named Entities (NEs) in a document
and to categorize them into certain Named entity classes such as the name of organization, person,
location, sport, river, city, country, quantity etc. In English, we have accomplished lot of work related to
NER. But, at present, still we have not been able to achieve much of the success pertaining to NER in the
Indian languages. The following paper discusses about NER, the various approaches of NER, Performance
Metrics, the challenges in NER in the Indian languages and finally some of the results that have been
achieved by performing NER in Hindi by aggregating approaches such as Rule based heuristics and
Hidden Markov Model (HMM).
KEYWORDS
HMM, Accuracy, NER, Performance Metrics, Named Entities
1. INTRODUCTION
There are numerous applications of Named Entity Recognition (NER).Some of these include:
Information Extraction, Question Answering, Information Retrieval, Automatic Summarization,
Machine Translation etc. The Named Entities can be known to us, if we perform computations on
the natural language. The task of extracting necessary details and retrieving important information
can be made easier and faster, if the Named entities are already known to us. NER is the process
in which Named Entities are detected in a document and are classified into their respective
Named Entity classes using any of the NER based approaches. According to the 8th
schedule,
India is known to have 22 official Indian languages. NER in Indian languages is still considered
to be a budding topic of research in the field of NLP and much of work is needed to be performed
in this regard.
Consider an example of NER in Hindi as follows:
“Mohit/PER ne/O mi road/LOC se/O kitab/O khareedi/O |/O
In the above sentence, the task of a NER based system is to extract and then classify the named
entities into certain classes. Here, we have considered ‘Mohit’ as the name of a person, so it is

44
shown by a PER tag. ‘mi road’ is the name of a location, so we have allotted a LOC tag to it. The
named entity tags that we choose may vary every time. It depends on the individual choice and
the contents that we have considered for the Named Entity Recognition. TABLE I lists some of
the Named Entity Tags. Named Entity tags may be of the general type or may further be
divided into sub tags which are of specific types. E.g. location tag (LOC) may further be
classified into continent tag, country tag, city tag, state tag, town tag, street tag etc.
Figure1. A single Named Entity tag split into more specific Named Entity tags
Table 1
Various Named Entity Tags. NE Tags: Named Entity Tags
PER: Name of Person, CO-Country, ORG-Organization, VEH-vehicle and QTY-Quantity
NE TAG EXAMPLE
PER Deepti, Sudha, Rohit
CITY Jaipur, Mumbai, Kolkata
CO India, China, Pakistan
STATE Rajasthan, Maharashtra
SPORT Hockey, Badminton
ORG TCS, Infosys, Accenture
RIVER Ganga, Krishna, kaveri
DATE 27-04-2012, 31/01/1989
TIME 10:10
PERCENT 100%
2. METHODOLOGIES FOR NER
There are basically two approaches that are employed in Named Entity Recognition. [5] [1] [18]
These include: Rule Based Approach and Machine learning based Approach [11] [6] [16].
2.1. Rule based Approach
It is also known as handcrafted approach. It is of two types:
LOCATION
CONTINENT
COUNTRY
STATE CITY
TOWN
STREET
T

45
2.1.1 List Lookup Approach
In this approach, Gazetteers are used that consists of different lists of Named Entity
classes and a simple look up operation is performed to conclude whether a word is a
Named Entity or not. If a particular word is found in a Named Entity class, then a Named
Entity tag is allotted to that word according to the Named Entity class in which it is
found. Indian languages lack in resources.
We can prepare Gazetteers in Indian languages using transliteration that would convert
English Named Entities into Indian languages. Some seed values of a domain specific
corpus can be used that would learn the context patterns and then Named Entities are
produced by the concept of bootstrapping.[17]This methodology is easy and fast .The
disadvantage of this approach is that it cannot overcome the problem of ambiguities.
E.g In a sentence:-““Ganga/PER Ne/O Ganga/RIVER nadi/O mein/O dupki/O Lagayi/O
|/O””. In this sentence, Ganga is a Named Entity .But, it can be a person name or a river
name .The ambiguity cannot be resolved by this methodology.
2.1.2. Linguistic Approach
In this approach, a linguist, who has an in depth knowledge about the grammar of specific
language constructs some rules, so that the Named Entities can be recognized as well as classified
easily. [3][20][19]The rules that are constructed are language independent and cannot be used to
identify Named Entities in some other language. [11]
2.2. Machine Learning Based Approach
This approach is also known as automated approach or Statistical approach. Machine learning
based approach is more efficiently and frequently used as compared to the Rule based approach.
2.2.1. Hidden Markov Model (HMM)
HMM is a statistical based approach in which states are hidden or unobserved .The HMM
produces sequence of tokens that are nothing but optimal state sequence.
It is based on the Markov Chain Property i.e. the probability of occurrence of the next state is
dependent on the just previous state. HMM is easy to implement. The disadvantage of this
approach is that it requires lot of training in order to get better results and it cannot be used for
large dependencies. [12]
Figure 2: Diagrammatic description of HMM

46
2.2.2. Maximum Entropy Markov Model (MEMM)
It combines the concept of Hidden Markov Model and Maximum Entropy Model. While training,
this model makes sure that the unknown values in a Markov Chain are connected and are not
conditionally independent of each other.
The large dependency problem of HMM is resolved by this model. Also, it has higher recall and
precision as compared to HMM. The disadvantage of this approach is the label bias problem. The
probabilities of transition from a particular state must sum to one. MEMM favours those states
through which less number of transitions occurs. [16]
Figure 3: Diagrammatic description of MEMM
2.2.3. Conditional Random Field (CRF)
It is graphical undirected model .Unlike other classifiers, it also takes into consideration the
context information or the neighbouring samples. It is known as Random field since it computes
the conditional probability on the following node given the present node values.
This methodology has advantages same as that of MEMM. Also it resolves the label bias problem
faced by MEMM. [3]
Figure 4.Diagrammatic description of CRF
MEMM
BEAM
SEARCH
HANDLING
UNKNOWN ENTITIES
GAZETTEER
POS TAGGED
TEXT
FINAL OUTPUT
UNTAGGED
TEXT
CRF
FORWARD
VITERBI &
BBACKWARD
A* SEARCH
HANDLING
UNKNOWN ENTITIES
GAZETTEER
POS TAGGED
TEXT
FINAL OUTPUT
UNTAGGED
TEXT

47
2.2.4. Support Vector Machine (SVM)
This methodology was introduced by Vapnik. SVM is a supervised statistical approach. The main
objective of this approach is to find whether a specific vector belongs to a particular target class
or not. [2] In this approach, the training as well as the testing data belongs to the single dimension
vector space.
Figure 5: Diagrammatic description of SVM
During training in this approach, we generate a hyper plane that is used to categorize the members
into two classes (positive and negative classes) that exists on the opposite sides of a hyper plane.
This approach also computes the distance of every vector from the hyper plane known as margin.
The main advantage of this approach is that it gives high accuracy for the text categorization
problem. [4]
2.2.5 Decision Tree
It is a well known methodology that is used to extract and categorize Named Entities in a given
corpus .In this approach, some recognition rules are applied to the untagged training corpus so
that Named Entities are retrieved. Now, we match these Named Entities obtained with the actual
answer key provided by the humans. If the Named Entity is same as the answer key, then it is
referred to as the positive example else it is known as negative example. [7]. A decision tree is
build that classifies the Named Entities in the testing document.[9] The leaf node of decision tree
depicts the resultant value of test .
3. PERFORMANCE METRICS
Performance Metrics is very important since it reveals the performance of a Named Entity
Recognition based system in terms of Precision, Accuracy and F-Measure. The output of a NER
system may be termed as “response” and the interpretation of human as the “answer key”. We
consider the following terms:
1. Correct-If the response is same as the answer key.
2. Incorrect-If the response is not same as the answer key.
3. Missing-If answer key is found to be tagged but response is not tagged.
4. Spurious-If response is found to be tagged but answer key is not tagged.[6]
Hence, we define Precision, Recall and F-Measure as follows:
SVM
BEAM
SEARCH
HANDLING
UNKNOWN ENTITIES
GAZETTEER
POS TAGGED
TEXT
FINAL OUTPUT
UNTAGGED
TEXT

48
Precision (P): Correct / (Correct + Incorrect + Missing)
Recall (R): Correct/ (Correct + Incorrect + Spurious)
F-Measure: (2*P*R)/(P+R) [5][8]
4. ISSUES IN NER IN INDIAN LANGUAGES
We still have not performed much of the work in NER in the Indian languages. This is mainly due
to the fact that Indian languages lack in resources such as annotated corpora and lexical resources.
There are many challenges related to the Named Entity Recognition in the Indian languages
.Some of them include the following:[6]
1. Lack of Capitalization: In Indian languages, the Capitalization concept is absent. Whereas, in
English and in many of the European languages, the word in which first alphabet is capital is a
proper noun. The NER based systems that are developed for the English and the European
languages, henceforth cannot be used to perform named entity recognition in the Indian languages
.Thus there is a need to develop an efficient NER based system for the Indian languages. [15]
2. Indian languages are inflectional and morphologically rich and are free word order.
3. Indian languages lack in resources .This problem is due to the fact that web mostly have lists of
Named Entities which are in English and not in the Indian languages.[17].
4. In dictionary of the Indian languages, many common nouns also exists as proper nouns. E.g.
Lata, Suraj, Aakash , Tara etc. are the Name of persons and common nouns as well. So, we need
to resolve ambiguities, which is also one of the issues in NER in the Indian languages
5. RESULTS
We have prepared a general corpus from the Hindi newspapers on the web. We have annotated it
manually. The Named Entity tags that we have used are: PER (Name of Person), LOC (Name of
Location), TIME, MONTH, SPORT, ORG (Name of Organization), VEH (Name of Vehicle),
RIVER and QTY (Quantity).In the first phase, we have applied the Rule based heuristics or the
shallow parsing technique over the Corpus, in which some of the helping words are used to detect
the Named Entities, that occur just after or before the Named Entities to be identified. In the
second phase, we apply Hidden Markov Model (HMM) to detect the rest of the Named Entities.

49
Table 2 Results of Rule based heuristics or shallow parsing technique
NAMED
ENTITIES
TOTAL NAMED
ENTITIES(NEs)
NAMED ENTITIES
(NEs) IDENTIFIED
ACCURACY
LOC 247 125 50.60%
PER 56 29 51.79%
QTY 79 40 50.63%
TIME 67 34 50.75%
ORG 135 68 50.37%
SPORT 45 23 51.11%
RIVER 11 6 54.54%
VEH 25 0 0%
MONTH 22 0 0%
TOTAL NEs = 687 TOTAL NEs DETECTED
= 325
TOTAL ACCURACY
= 47.5%
Table 3 Results of Hidden Markov Model (HMM)
NAMED
ENTITIES
TOTAL NAMED
ENTITIES (NEs)
UNDETECTED
NAMED ENTITIES (NEs)
IDENTIFIED
ACCURACY
LOC 122 107 87.70%
PER 27 24 88.89%
QTY 39 34 87.18%
TIME 33 29 87.88%
ORG 67 59 88.06%
SPORT 22 20 90.90%
RIVER 5 5 100%
VEH 25 25 100%
MONTH 22 22 100%
TOTAL NEs = 362 TOTAL NEs DETECTED
= 325
TOTAL ACCURACY
= 89.78%
Table 4 Results of Combination of Approaches or Hybrid Approach
TOTAL NAMED
ENTITIES (NEs)
NAMED ENTITIES (NEs)
IDENTIFIED
ACCURACY
687 650 94.61%

Figure 6 Results of using Combined Approach
6. CONCLUSIONS
We have obtained accuracy of about 94.61% by
HMM, as shown in Table 4. Table 2 depicts that if we applied only Rule Based Heuristics, then it
performed very poorly, and the accuracy
depicts that if we applied only HMM, then it
obtained by this approach was
combined approach, then it gives very good results in
ACKNOWLEDGEMENT
I would like to thank all those who helped me in accomplishing this task.
REFERENCES
[1] Animesh Nayan,, B. Ravi Kiran Rao, Pawandeep Singh,Sudip Sanyal and Ratna Sanya “Named
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[2] Asif Ekbal and Sivaji Bandyopadhyay. “
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90
91
92
93
94
95
96
97
98
99
100
101
Figure 6 Results of using Combined Approach
We have obtained accuracy of about 94.61% by aggregating the rule based heuristic
Table 2 depicts that if we applied only Rule Based Heuristics, then it
performed very poorly, and the accuracy obtained by this approach was 47.5%. Similarly, Table 3
we applied only HMM, then its performance was average, and the accuracy
obtained by this approach was 89.78%. This shows that if we apply hybrid approach
combined approach, then it gives very good results in a Named Entity Recognition based system
I would like to thank all those who helped me in accomplishing this task.
Animesh Nayan,, B. Ravi Kiran Rao, Pawandeep Singh,Sudip Sanyal and Ratna Sanya “Named
Entity Recognition for Indian Languages” .In Proceedings of the IJCNLP-08 Workshop on NER for
South and South East Asian Languages ,Hyderabad (India) pp. 97–104, 2008.
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58, Hyderabad, India, January 2008..
NAMED ENTITIES
LOC
PER
QTY
TIME
ORG
SPORT
RIVER
VEH
MONTH
50
le based heuristics and the
Table 2 depicts that if we applied only Rule Based Heuristics, then it
Similarly, Table 3
, and the accuracy
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Asif Ekbal and Sivaji Bandyopadhyay 2008 “ Bengali Named Entity Recognition using Support
Workshop on NER for South and South East Asian

51
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52
Authors
Deepti Chopra received B.Tech degree in Computer Science and Engineering from
Rajasthan College of Engineering for Women, Jaipur, Rajasthan in 2011.Currently she is
pursuing her M.Tech degree in Computer Science and Engineering from Banasthali
University, Rajasthan. Her research interests include Artificial Intelligence, Natural
Language Processing, and Information Retrieval.
Nusrat Jahan received B.Tech degree in Computer Science and Engineering from R.N.
Modi Engineering College, Kota, Rajasthan in 2010.Currently she is pursuing her
M.Tech degree in Computer Science and Engineering from Banasthali University,
Rajasthan. Her research interests include Artificial Intelligence, Natural Language
Processing, and Information Retrieval.
Sudha Morwal is an active researcher in the field of Natural Language Processing.
Currently working as Associate Professor in the Department of Computer Science at
Banasthali University (Rajasthan), India. She has done M.Tech (Computer Science) ,
NET, M.Sc (Computer Science) and her PhD is in progress from Banasthali University
(Rajasthan), India.

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