Assessing the Impact of Stemming Algorithms Applied to Judicial

Jurisprudence

An Experimental Analysis

Robert A. N. de Oliveira and Methanias Colac¸o J

unior

UFS - Universidade Federal de Sergipe, S

ao Crist

ao, SE, Brazil

Keywords:

Dimensionality Reduction, Experimental Analysis, Jurisprudence, Stemming.

Abstract:

Stemming algorithms are commonly used during textual preprocessing phase in order to reduce data dimen-

sionality. However, this reduction presents different efﬁcacy levels depending on the domain that it’s applied

to. Hence, this work is an experimental analysis about dimensionality reduction by stemming a real database

of judicial jurisprudence formed by four subsets of documents. With such document base, it is necessary to

adopt techniques that increase the efﬁciency of storage and search for such information, otherwise there is a

loss of both computing resources and access to justice, as stakeholders may not ﬁnd the document they need to

plead their rights. The results show that, depending on the algorithm and the collection, there may be a reduc-

tion of up to 52% of these terms in the documents. Furthermore, we have found a strong correlation between

the reduction percentage and the quantity of unique terms in the original document. This way, RSLP algorithm

was the most effective in terms of dimensionality reduction, among the stemming algorithms analyzed, in the

four collections studied and it excelled when applied to judgments of Appeals Court.

1 INTRODUCTION

Every day, the courts, through their magistrates, judge

various themes of the Law, generating a large base of

legal knowledge that guides new decisions and works

as an argumentative base to the related parties that

plead their interests. Hence, from the corpus formed

by a uniform set of decisions handed down by the ju-

diciary on a particular subject (Maximiliano, 2011),

emerges the concept of jurisprudence, fundamental

tool for legal professionals to exercise their role.

This way, those decisions generate three types of

documents (Santos, 2001):

• Trial Court sentence: when the judge utters a pro-

cedural trial in ﬁrst instance;

• Monocratic Decision: when a magistrate decides

alone, in second instance, a lawsuit that has uni-

form interpretation;

• Judgment: when collegiate organ, composed by

one rapporteur and at least two magistrates, utters

sentence in second instance.

A decision in second instance may be the re-

sult of an appeal from a sentence uttered by an Ap-

peals Court judge or by Special Courts judge, creating

speciﬁcs documents for each one of them.

With such document base, it is necessary to adopt

techniques that increase the efﬁciency of storage and

search for such information, otherwise there is a loss

of both computing resources and access to justice, as

stakeholders may not ﬁnd the document they need to

plead their rights.

In this scenario, according to (Flores and Mor-

eira, 2016; Orengo et al., 2007), stemming algorithms

can reduce the texts dimensionality, thereby improv-

ing the use of computing resources, and increase the

relevancy of the results returned by retrieval systems.

In fact, these algorithms are commonly used during

textual preprocessing phase in order to reduce data

dimensionality. However, this reduction presents dif-

ferent efﬁcacy levels depending on the domain it is

applied. The legal universe has its own jargon and we

have not found reports in the literature showing that

the same beneﬁts are obtained when stemming is ap-

plied to jurisprudential bases.

Therefore, the objective of this study was to ana-

lyze, following an experimental process, using quan-

titative metrics, the effectiveness of stemming on the

dimensionality reduction of real jurisprudential bases.

The results showed that, depending on the algorithm

and the collection, there may be a reduction of up to

52% of these terms in the documents. Furthermore,

Oliveira, R. and Júnior, M.

Assessing the Impact of Stemming Algorithms Applied to Judicial Jurisprudence - An Experimental Analysis.

DOI: 10.5220/0006317100990105

In Proceedings of the 19th International Conference on Enterprise Information Systems (ICEIS 2017) - Volume 1, pages 99-105

ISBN: 978-989-758-247-9

we have found a strong correlation between the re-

duction percentage and the quantity of unique terms

in the original document. This way, RSLP algorithm

was the most effective in terms of dimensionality re-

duction in the four collections analyzed and it was ex-

celled when applied to judgments of Appeals Court.

The rest of the paper is structured as follows. Sec-

tion 2 presents the related work. Section 3 concep-

tualizes stemming and describes the algorithms used

in this research. In Section 4, we present the deﬁ-

nition and planning of the experiment. In Section 5,

we show the experiment execution. Section 6 con-

tains the results of the experiment. Finally, Section 7

presents the conclusion and future work.

2 RELATED WORK

This paper analyses the impact of stemming on di-

mension reduction of jurisprudence texts in Brazilian

Portuguese, therefore this section will present articles

that had a similar approach.

(Alvares et al., 2005) carried out an assessment of

vocabulary reduction, along with overstemming and

understemming errors described in the following sec-

tion , by stemming 1,500 words available in dictionar-

ies of Brazilian Portuguese language. This approach

differs from ours, since they propose a new stemming

algorithm, StemBR, and compares it to two different

ones. On the other hand, here we will use algorithms

available in (Lucene, 2005).

(Orengo et al., 2007) conducted a comparative

study of stemming algorithm related to reduction of

terms in a collection of tests formed by the Folha

de S

ao Paulo newspaper and evaluated its impact on

the results returned by a retrieval system. Different

from this proposal, there are no further details on the

dimensionality reduction per document, considering

that they focused on an analysis of the metrics taken

from the search system.

Similar to the article mentioned above, (Flores

and Moreira, 2016) measured the impact of stemming

on testing collections available in different languages

(English, French, Portuguese and Spanish). This way,

they collected dimensionality reduction metrics, over-

stemming, understemming and also measured the re-

ﬂection on the application of these algorithms in pre-

cision and recall of information retrieval systems.

However, due to its scope, the paper did not go into

detail on any of the analyzes.

It is worth mentioning that, until now, papers that

run a detailed analysis of dimensionality reduction per

document, like the one presented, were not found. In

addition, related work used collections that do not re-

ﬂect the documents found in the legal universe.

3 STEMMING

The stemming process consists of grouping different

words connected by a common stem, based on a set

of rules which act by removing sufﬁxes and preﬁxes

(Figure 1). Table 1 shows the application of ﬁve stem-

ming algorithms used during this experiment with six

distinct words, in which NoStem is the control group,

i.e., it generates no reduction of terms.

Except for the control group, the other algorithms

used in the experiment are based on rules and act by

removing sufﬁxes (Flores and Moreira, 2016):

• Porter: originally written in English, in 1980, and

adapted to Portuguese language later;

• RSLP (Removedor de Suﬁxos da Lingua Por-

tuguesa): published in 2001, contains approxi-

mately 200 rules and an exception list to almost

each one of them;

• RSLP-S: a lean version of RSLP that uses only

plural reduction;

• UniNE: contains less rules than Porter and RSLP,

however it is more aggressive than RSLP-S.

Figure 1: Sequence of steps for the RSLP algorithm(Orengo

et al., 2007).

Thus, considering semantic and morphological as-

pects, a stemming algorithm can commit two error

types: a) overstemming, when the part removed is not

a sufﬁx, instead it is part of the word stem; and b) un-

derstemming, when the sufﬁx removal does not take

place fully. In this study, such errors will not be eval-

uated.

ICEIS 2017 - 19th International Conference on Enterprise Information Systems

100

Table 1: Example of stemming using ﬁve algorithms of the experiment.

NoStem constituic¸

oes limitac¸

oes regimento considerando anu

encia estelionato

Porter constituic¸

o limit regiment consider anu

enc estelionat

RSLP constitu limit reg consider anu estelionat

RSLP-S constituic¸

ao limitac¸

ao regimento considerando anu

enc estelionato

UniNE constituica limitaca regiment considerand anuenci estelionat

4 DEFINITION AND

EXPERIMENT PLANNING

In this and next two sections, this paper will be

presented as an experimental process according to

Wohlin et al. guidelines, described in (Wohlin et al.,

2012). Therefore, initially, we will explain planning

and deﬁnition of the experiment. After that, we will

refer to its execution and data analysis.

4.1 Goal Deﬁnition

The goal of this work is to analyze the impact of stem-

ming algorithms in the dimensionality reduction of ju-

risprudential documents.

In order to achieve it, we will conduct an exper-

iment, in a controlled environment, in which the re-

duction of unique terms per document will be mea-

sured, inside each collection, along with an analy-

sis of statistically signiﬁcant differences of effective-

ness of the same algorithm, among four documentary

bases adopted by the study.

The following is the goal formalization, accord-

ing to GQM model proposed by Basili (Basili et al.,

1994): Analyze stemming algorithms with the pur-

pose of evaluating them with respect to dimension-

ality reduction and effectiveness from the point of

view of data analysts in the context of jurispruden-

tial documents.

4.2 Planning

Context Selection. The experiment will be in vitro

and will use the entire judicial jurisprudence database

of Supreme Court of the State of Sergipe, formed

by four collections: a) judgments of Appeals Court

(181,994 documents); b) monocratic decisions of Ap-

peals Court (37,142 documents); c) judgments of Spe-

cial Courts (37,161 documents); and d) monocratic

decisions of Special Courts (23,151 documents).

Dependent Variables. The average of unique terms

per document (UTD) and the average percentage of

reduction of unique terms per document (RP) taken

from the stemmer application.

• Unique Terms: UT D

= Frequency of unique

terms after document stemming.

• Average of unique terms: µ = (UT D

UT D

+ +UT D

)/n

• Reduction percentage: RP

= 100 − (UT D

∗

100)/UT D

NoStem

• Average of reduction percentage: µ = (RP

+ ... + RP

)/n

Independent Variables. Document collection of

judgments of Appeals Court (JAC), monocratic deci-

sions of Appeals Court (MAC), judgments of Special

Courts (JSC) monocratic decisions of Special Courts

(MSC); the stemming algorithms (NoStem, Porter,

RSLP, RSLP-S and UniNE).

Hypothesis Formulation. The research questions for

this experiment are: do stemming algorithms reduce

the dimensionality of jurisprudential documents? Is

the effectiveness of each algorithm the same for all

four collections studied?

For the ﬁrst research question, we considered the

quantity of unique terms per document as a metric to

evaluate the dimensionality reduction. For the second

question, we adopted the reduction percentage of each

algorithm, considering that the comparison was made

among documents of a different nature, making the

use of absolute values inadequate. In this scenario,

the following assumptions will be veriﬁed:

Hypothesis 1 (For each of the four collections).

• Null Hypothesis H0

UTD

: The stemming algo-

rithms have the same average of unique terms per

document (µ

NoStem

UT D

= µ

Porter

UT D

= µ

RSLP

UT D

RSLP−S

UT D

= µ

UniNE

UT D

• Alternative Hypothesis H1

UTD

: The stemming

algorithms have different averages of unique

terms per document (µ

UT D

6= µ

UT D

for at least one

pair(i,j)).

Hypothesis 2 (For each of the stemming algo-

rithms).

• Null Hypothesis H0

: The percentage averages

of reduction of unique terms per document are the

same in all four collections (µ

JAC

= µ

MAC

JSC

= µ

MSC

Assessing the Impact of Stemming Algorithms Applied to Judicial Jurisprudence - An Experimental Analysis

101

• Alternative Hypothesis H1

: The percentage

averages of reduction of unique terms per docu-

ment are different in all four collections (µ

for at least one pair(i,j)).

Selection of Participants and Objects. The docu-

ments of each collection were chosen randomly tak-

ing into consideration their number of characters. So,

the quantity of documents were determined by the

sample calculation of a ﬁnite population:

n =

.σ

.(N − 1).z

.σ

(1)

Where, n is the sample size, z is the standardized

value (we adopted 1.96, i.e., 95% of trust level), σ is

the standard deviation of population, e is the margin

of error (we adopted 5% of σ) and N is the population

size. Table 2 shows the number of selected documents

after sample calculation, along with size, mean and

standard deviations of the population.

Table 2: Sample size per collection.

Coll. N µ σ n

JAC 181,994 11,626.65 8,270.08 1,524

MAC 37,142 8,396.27 6,940.01 1,476

JSC 37,161 9,509.41 5,718.97 1,476

MSC 23,151 6,569.90 4,009.80 1,442

Experiment Project. The jurisprudential documents

have a great variability in terms of number of charac-

ters, thus, in order to ensure conﬁdence on hypothesis

tests, we will utilize a randomized complete block de-

sign (RCBD) (Wohlin et al., 2012) , this way, each

algorithm will be applied to the same document and

those documents will be randomly taken from each

collection, increasing the experiment precision. Fur-

thermore, before applying stemming, a preprocessing

for textual standardization will be performed in which

the content of documents will be shifted to small caps

and punctuation characters will be removed. NoStem

represents the unique terms of the document with no

stemming, therefore, it acts as a control group.

Instrumentation. We developed a Java application

in order to iterate on each document of the sample,

applying stemming algorithms and counting the fre-

quency of unique terms after the execution. In the

end, the application will store the observations per-

formed in a CSV ﬁle (Comma Separated Values) for

each collection.

5 EXPERIMENT EXECUTION

5.1 Preparation

The preparation phase consisted of obtaining collec-

tions referring to judicial jurisprudence. Thus, doc-

uments were extracted from an OLTP base (Online

Transaction Processing) and converted to XML for-

mat (eXtensible Markup Language) facilitating the

experiment packaging.

5.2 Execution

By the end of previous phases, the experiment started

executing the Java application, in accordance with

what was deﬁned in the planning phase.

5.3 Data Collection

The application recorded, for each collection, the doc-

ument identiﬁer, the number of unique terms and the

stemming algorithm adopted CSV format (Table 3).

Table 3: Input example in CSV ﬁle.

ID,UTD,Stemmer

201100205001443632662,679,NoStem

201100205001443632662,580,Porter

201100205001443632662,547,RSLP

201100205001443632662,651,RSLPS

201100205001443632662,636,UniNE

5.4 Data Validation

The Java application was built using Test Driven

Development (TDD) (Agarwal and Deep, 2014) ap-

proach , therefore, we wrote unit test cases to validate

if the frequency count of unique terms per document

worked as expected.

Averages of unique terms per document were

computed and the percentage averages of dimension-

ality reduction were obtained by applying stemming

algorithms, considering control group.

To support this analysis, interpretation and results

validation, we used ﬁve types of statistical tests: the

Shapiro-Wilk test, the Friedman test, the Kruskal-

Wallis test, the Wilcoxon test and the Mann-Whitney

test. The Shapiro-Wilk test was used to verify sam-

pling normality, as literature shows it has higher

test power than other approaches (Ahad et al., 2011;

Razali and Wah, 2011). Considering RCBD project of

the experiment, with a factor and multiple treatments,

the Friedman test (Theodorsson-Norheim, 1987) and

the Kruskal-Wallis test (Wohlin et al., 2012) were

ICEIS 2017 - 19th International Conference on Enterprise Information Systems

102

used to demonstrate the existence of different aver-

ages of paired and independent samples, respectively,

that did not obtain data normality, verifying χ

(Chi-

Square) magnitude. Finally, a post hoc analysis of

the Friedman and Kruskal-Wallis tests was run us-

ing, respectively, the Wilcoxon and Mann-Whitney

tests, to compare the averages of each treatment, ap-

plying the Benferroni adjustment in the signiﬁcance

level (Holm, 1979). As we perform multiple compar-

isons among different treatments, this adjustment is

important, since it reduces the possibility of rejection

of the null hypothesis when it is indeed true (Error

Type I) (Dunn, 1961).

All statistical tests were performed using SPSS

(SPSS, 2012) and re-evaluated with R (Team, 2008)

and SciPy (Jones et al., 2001).

6 RESULTS

To answer experimental questions, CSV ﬁles gener-

ated by the Java application were analyzed. The re-

sults of stemming impact on the average of unique

terms per document and on percentage average of di-

mensionality reduction per document, can be seen in

Figure 2 and Figure 3, respectively.

Figure 2: The average number of unique terms per docu-

ment obtained by each stemmer.

6.1 Analysis and Interpretation

Visually, analyzing Figures 2 and 3, a stemming ap-

plication seems to generate differences in both, the

average of reduction of unique terms per document

and in the average percentage of dimensionality re-

duction. However, it is not possible to claim that with

no statistical evidences that conﬁrm that.

Finally, we used 95% of trust level (α = 0.05), to

the entire experiment and, later on, we analyzed if

the samples had normal distribution. However, this

Figure 3: The average percentage of dimensionality reduc-

tion per document generated by stemming.

hypothesis was rejected, since the Shapiro-Wilk test

obtained p-value below 0.001, lower than the signif-

icance level adopted, in every collection and algo-

rithm. This way, considering data distribution and

RCBD design adopted for the experiment, we per-

formed the Friedman test to verify Hypothesis 1 (Ta-

ble 4).

Table 4: Results of the Friedman tests for the Hypothesis 1.

Coll. χ

p-value

JAC 5,883.84 0.000

MAC 5,590.32 0.000

JSC 5,863.67 0.000

MSC 5,474.95 0.000

After applying the tests, we found a strong ev-

idence for the hypothesis H1

UTD

, showing that the

averages of unique terms per document are not the

same among the algorithms, since we veriﬁed a p-

value below 0.001, to every collection, and χ

equal to

5,883.84; 5,590.32; 5,863.67 and 5,474.95, referred

to collections JAC, MAC, JSC and MSC, respectively.

After a post-hoc analysis with the Wilcoxon test, ap-

plying the Benferroni correction (α = α / 10), we

found the following order related to the number of

unique terms obtained after stemming: NoStem >

RSLP-S > UniNE > Porter > RSLP, to every collec-

tion. In other words, RSLP algorithm was the most

effective in the reduction of unique terms per docu-

ment.

For Hypothesis 2, considering that the jurispru-

dential bases are independent, i.e., the same docu-

ment does not appear in more than one collection, we

adopted Kruskal-Wallis tests (Table 5).

According to the results, the percentage averages

of reduction of algorithms are not the same for every

collection, since p-value was less than 0.001 and χ

equal to 687.93; 711.83; 250.31 and 295.25, referred,

Assessing the Impact of Stemming Algorithms Applied to Judicial Jurisprudence - An Experimental Analysis

103

Figure 4: Correlation matrix among stemming algorithms.

NoStem unit is UTD and others are RP.

Table 5: Results of the Kruskal-Wallis tests for the Hypoth-

esis 2.

Stemmer χ

p-value

Porter 687.93 0.000

RSLP 711.83 0.000

RSLP-S 250.31 0.000

UniNE 295.25 0.000

respectively, to Porter, RSLP, RSLP-S and UniNE al-

gorithms, therefore, hypothesis H0

was refuted. By

conducting a post-hoc with the Mann-Whitney test,

also applying the Benferroni adjustment (α = α / 6),

we noticed that stemming algorithms reduced dimen-

sionality more effectively in JAC collection.

As it can be seen in the ﬁrst line of the correla-

tion matrix showed by Figure 4, there is a strong pos-

itive correlation, ranging from 0.70 to 0.89, between

the quantity of unique terms per document and the re-

duction percentage achieved by stemming algorithms.

In other words, it suggests that the more words ju-

risprudential documents have, the better results the

analyzed stemming algorithms will get. Furthermore,

in the same ﬁgure, we noticed a linear relation be-

tween the algorithms, indicating that they maintain a

proportionality related to the potential of dimension-

ality reduction of texts. Thus, the Porter and RSLP

algorithms, for example, have a 0.97 correlation co-

efﬁcient, indicating an almost perfect positive linear

relationship.

To illustrate this correlation potential between

quantity of unique terms and reduction percentage,

we considered the entire sample of each collection as

Table 6: Sample dimensionality reduction.

Coll. Porter RSLP RSLP-S UniNE

JAC 46% 52% 12% 24%

MAC 39% 45% 11% 22%

JSC 35% 41% 10% 20%

MSC 35% 41% 10% 19%

a single document. Then, we applied stemming algo-

rithms to the collection.

In this scenario, shown in Table 6, one of the stem-

ming algorithms achieved 52% of reduction (JAC-

RSLP), conﬁrming the linear relation mentioned

above. We also noticed that the order of effectiveness

was equivalent to the one found in the experiment us-

ing single documents (RSLP > Porter > UniNE >

RSLP-S > NoStem).

Hence, due to the results found, it is possible to

say that RSLP algorithm reduced judicial jurispru-

dence dimensionality more effectively than Porter,

UniNE and RSLP-S. Besides, JAC collection showed

higher reduction of unique terms, regardless which

stemming algorithm was adopted.

6.2 Threats to Validity

Because the data was collected and analyzed by the

authors, there happens to be a strong threat to internal

and external validities. However, there is not conﬂict

of interest. Thus, there are no reasons to privilege

an algorithm over another. To mitigate any possible

bias, documents were chosen randomly, according to

RCBD guidelines.

7 CONCLUSION AND FUTURE

WORK

This paper showed an important contribution related

to application of stemming algorithms on jurispruden-

tial bases. Indeed, data dimensionality reduction is

used in a variety of text processing techniques, how-

ever, we have not found, so far, a quantitative study

that analyzes its impact on Brazilian judicial real de-

cisions.

According to experimental results, the use of

stemming algorithms reduced the average of unique

terms per document by 52%. Furthermore, we have

found a strong correlation between the reduction per-

centage and the quantity of unique terms in the orig-

inal document. This way, among the stemming al-

gorithms analyzed, RSLP was the most effective in

terms of dimensionality reduction in the four collec-

tions studied and it was excelled when applied to

ICEIS 2017 - 19th International Conference on Enterprise Information Systems

104

judgments of Appeals Court.

Finally, for future work, we intend to analyze the

reﬂection of the reduction from the perspective of a

judicial information retrieval system, measuring its

impact on MAP, R-Precision and Pr@10 metrics.

ACKNOWLEDGEMENTS

This study counted on Supreme Court of the State of

Sergipe full support by sharing their database of judi-

cial jurisprudence in text format.

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