Improvement of effort estimation accuracy in software projects using a feature selection approach
Subject Areas : Data MiningZahra Shahpar 1 , Vahid Khatibi 2 , Asma Tanavar 3 , Rahil Sarikhani 4
1 - Department of Computer Engineering, Kerman Branch, Islamic Azad University, Kerman,Iran.
2 - Faculty Member of Islamic Azad University, Kerman Branch, Kerman,Iran.
3 - Department of Computer, Kerman Branch, Islamic Azad University
4 - Department of Computer, Kerman Branch, Islamic Azad University, Iran
Keywords:
Abstract :
[1] Y. S. Seo, et al, “AREION: Software effort estimation based on multiple regressions with adaptive recursive data partitioning”, ELSEVIER, Information and Software Technology, vol.55, pp. 1710-7725, 2013.
[2] A. S. Grewal, et al, “Emerging Estimation Techniques”, International Journal of Computer Applications (0975 – 8887), vol. 52, no. 8, pp. 30–34, 2012.
[3] Hatami, Nafiseh, “Examination of Feature Selection Based Methods”, ict center, Malek-Ashtar University of Technology, 2013.
[4] E. Papatheocharous, et al, “Feature Subset Selection for Software Cost Modelling and Estimation”, 2010.
[5] V. Khatibi, et al, “Increasing the Accuracy of Analogy Based Software Development Effort Estimation Using Neural Networks”, International Journal of Computer and Communication Engineering, Vol. 2, No. 1, pp. 78-81, 2013.
[6] A. Zaid, et al, “Issues in Software Cost Estimation,” International Journal of Computer Science and Network Security, vol. 8, no. 11, pp. 350–356, 2008.
[7] F. Ferrucci, et al, “Genetic Programming for Effort Estimation:An Analysis of the Impact of Different Fitness Functions”, 2nd International Symposium on Search Based Software Engineering, PP. 89-91, IEEE, 2010.
[8] M. Singh, et al, “Software Productivity Empirical Model for Early Estimation of Development”, International Journal of Computer Science and Information Technologies, Vol. 5 (1), 2014, pp. 682-685, 2014.
[9] M. Azzeh, et al, “An Optimized Analogy-Based Project Effort Estimation”, International Journal of Advanced Computer Science and Applications, Vol.5, no.4, pp. 6-12, 2014.
[10] R. p, et al, “Building Software Cost Estimation Models using Homogenous Data”, IEEE, First International Symposium on Empirical Software Engineering and Measurement, PP.393-400, 2007.
[11] M. Karagiannopoulos, et al. “Feature Selection for Regression Problems”, Educational Software Development Laboratory, Department of Mathematics, University of Patras, Greece, 2004.
[12] H. Hamza, et al, “Software Effort Estimation using Artificial Neural Networks: A Survey of the Current Practices”, IEEE, 10th International Conference on Information Technology: New Generations, PP.731-733, 2013.
[13] M. Melanie, an Introduction to Genetic Algorithms, Cambridge, Massachusetts. London, England, Fifth printing, 1999.
[14] J. Han and M. Kamber, Data Mining:Concepts and Techniques, Second Edition, Elsevier, University of Illinois at Urbana-Champaign, 2006.
[15] M. O. Elish, et al, “Empirical Study of Homogeneous and Heterogeneous Ensemble Models for Software Development Effort Estimation”, Hindawi Publishing Corporation Mathematical Problems in Engineering,Vol 2013, Article ID 312067, 21 pages, http://dx.doi.org/10.1155/2013/312067, 2013.
[16] V. Khatibi, et al, “A New Fuzzy Clustering Based Method to Increase the Accuracy of Software Development Effort Estimation”, World Applied Sciences Journal, vol.14 (9), pp.265-1275, 2011.
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Journal of Advances in Computer Engineering and Technology
Improvement of effort estimation accuracy in software projects using a feature selection approach
Abstract— In recent years, utilization of feature selection techniques has become an essential requirement for processing and model construction in different scientific areas. In the field of software project effort estimation, the need to apply dimensionality reduction and feature selection methods has become an inevitable demand. The high volumes of data, costs, and time necessary for gathering data, and also the complexity of the models used for effort estimation are all reasons to use the methods mentioned. Therefore, in this article, a genetic algorithm has been used for feature selection in the field of software project effort estimation. This technique has been tested on well-known datasets. Implementation results indicate that the resulting subset, compared to the original dataset, has produced better outcomes in terms of effort estimation accuracy. This article showed that genetic algorithms are ideal methods for selecting a subset of features and improving effort estimation accuracy2.
I. INTRODUCTION
O
ne of the important and effective stages in a software engineering process which can play an important role in the success or failure of the project is effort and cost estimation [1]. The two phrases cost estimation and effort estimation are usually equivalently used in software engineering and project management surveys [2]. Accurate effort estimation for resource allocation and project planning is of great importance. Underestimating a software project
effort causes delays in project scheduling, increases costs, and eventually leads to the projects’ failure. On the other hand, overestimation of a project effort in effectively utilizing software resources has its own side effects [1].
Accurate estimation of a software project effort is a difficult task considering that multiple parameters are used in software project effort estimation. The data sets used are mostly multi-dimensional, which despite creating certain opportunities, also create many computational challenges. One of the existing problems in this regard is that not all features are critical for finding the hidden knowledge amongst the important data, and in many cases, some of the candidate features are unrelated and redundant. In addition, the gathering of these data is time consuming and highly costly. These unnecessary features dramatically reduce the algorithms learning speed and accuracy. Moreover, recent surveys have shown that data quality and the fitness of the datasets utilized in effort estimation techniques are key factors for achieving better results. Additionally, through selecting subsets from these features, we are able to reduce the model estimation complexity. Therefore, selecting related and necessary features is of fundamental importance for increasing model efficiency [3, 4].
Therefore, this article attempts to apply a feature selection method for improving accuracy and efficiency of effort estimation. We will present our proposed method for examining datasets in section 2, after reviewing the researches accomplished in the field of effort estimation and feature selection, and in the 3rd section, we will examine the methods evaluation measures, and in the 4th and final sections of the article, we will analyze the concluded results.
II. Review of literature
In this section, we will review background studies with respect to two approaches; software project effort estimation and feature selection:
The initial idea of effort estimation dates back to the 1950s. In 1965, with increases in software projects and demand for high quality software, regression techniques were deployed for effort estimation [5]. During 1970, the COCOMO model was formulated by Barry W.Boehm and C.Bats. During the 1980’s, many developments were made on effort estimation models and methods including the changes applied by Boehm et al on COCOMO, which resulted in the new model, COCOMO II [6]. Therefore, it can be said that during the last years, many studies were made in the field of effort estimation leading to increases in effort estimation accuracy including the following cases:
Filomena Ferrucci et al [7] used genetic programing for effort estimation and result analysis. They indicated that genetic programming, in comparison with other methods, increases estimation accuracy. Mandeep Singh et al [8] presented a practical model for early estimation of software development. After data analysis, they calculated the influence of different parameters on productivity. Mohammad Azzeh et al [9] used an artificial bee colony algorithm for determining the appropriate number and factor of each feature for software project effort estimation. They evaluated this method on 8 promising datasets. Rahul Premraj et al [10] presented a model for cost estimation using homogenous data.
On the other hand, feature selection has been examined in different perspectives at the hands of various authors. It can be said that the purpose of feature selection is to select a certain subset of features in order to increase effort estimation accuracy. In other words, reduction in structure size will occur without any significant reductions in estimation accuracy, which is obtained through utilizing the features presented [3].
Different feature selection methods can be categorized into various sets according to search methods. In some methods, the whole space possible is searched completely whereas in other methods, the search space may become smaller with a trade-off of losing a little efficiency [3]. Each of these methods can be categorized into different fields according to their application. In the following, some of the methods used in cost and effort estimation are mentioned.
Efi Papatheocharous et al. [4] examined four feature selection methods including stepwise regression Garson’s algorithm on artificial neural networks (ANN), forward selection, backward elimination, and genetic algorithm using a ridge regression and last squares technique on two datasets; Desharnais and ISBSG.
Karagiannopoulos et al. [11] compared five wrapper feature selection methods using regression algorithms. The wrapper approach is known as the black box method in which a scale function is used to evaluate the appropriateness of feature subsets. Methods used in this article include Forward Selection (FS), Backward Selection (BS), the Best First forward selection (BFFS), the Best First Backward Selection (BFBS), and Genetic Search Selection (GS). The regression algorithms used are: Regression Trees, Regression Rules, Instance-Based Learning Algorithms, and Support Vector Machines. Also, in order to execute the method and analyze the results, 12 uci datasets have been used in this article.
III. Methodology
As previously mentioned, this research was performed with the purpose of increasing effort estimation accuracy using feature selection. In this research, a genetic algorithm has been used for feature selection. So far only statistical techniques, regression, and evolutionary algorithms have been used. The bee colony algorithm has been used in numerous effort estimation scenarios for different software projects [5-10]. Also, different methods have been used for feature selection on different datasets. However, genetic algorithms have not been amongst these methods [3-4-11]. Thus, this study focuses on proposing a genetic algorithm for feature selection, which will be explained further in the next sections. Before applying this genetic algorithm on datasets, in order to prevent problems related to the huge differences of magnitudes, calculations, and the overflow of variables, the data have been normalized such that all independent variables in the used data sets have been mapped to a number within the interval of [0-1]. Then, the data have been categorized into testing and learning sets using a 3-fold standard method and the genetic algorithm has been implemented on the learning data. The working procedure is shown in figure 1. Therefore, after examining the evaluation method and the proposed performance evaluation method, we will describe and examine the genetic algorithm, cost function of the genetic algorithm, and the utilized data sets.
1. evaluation method
A 3-fold standard method has been used for result evaluation. In this method, the target dataset is first randomly divided into 3 approximately equal parts and each time one of these three parts is used as the testing data and the 2/3 left is used as the learning data for genetic algorithm cost function optimizations. This procedure is repeated 30 times and then the results are examined.
2. operational parameters
Many evaluation criteria have been defined and utilized in effort and cost estimation techniques. The four very common evaluation criteria used in studies are MRE which is the effort estimation difference error rate by the algorithm using real efforts, mean MRE (MMRE) which is the mean estimation error rate for all target samples (learning or testing), median MRE (MDMRE) or the median error rate of the amount determined by the algorithm using real effort samples, and finally PRED(x) which is the percentage of samples which had an error rate of less than or equal to x. These four parameters are calculated as follows [5, 12]:
(1)
in which Actual Effort is the amount of effort of the real target project within the dataset and Estimated Effort is the estimation effort evaluated by the algorithm [5].
(2)
In which n denotes the number of projects evaluated. Lower MMRE indicates lower estimation error of the algorithm, which implies better accuracy [12].
(3)
(4)
in which X is the target difference which in most studies is 0.25, K denotes the number of samples for which the difference of effort estimated by the algorithm along with the real effort are less than or equal to X, n is the total number of samples being evaluated [5].
3. Genetic Algorithm
The JACET algorithm was introduced by John Holland (1967). This method later became very popular with the help of Goldberg (1989). In this method, based on the gradual evolution theory and other fundamental ideas of Darwin, an initial set of target parameters is randomly produced for a constant number of samples namely the initial population. Then, the simulation program is executed and the number indicating the standard deviation or the practice of that information set is ascribed to the given population (fitness). This procedure is repeated for each and every member produced. Then, by calling the genetic algorithm operators including crossover and mutation, the next generation selection is formed and the routine is continued until the convergence criterion is met [13].
In this study, using a genetic algorithm, a random population with a constant size of 50 members from the candidate features is produced. The feature set presented or namely the chromosomes are shown as a binary string with length n in which a zero or one at location i of the string shows the presence or absence of feature i in the feature set selected. n is the total number of features available. In each iteration, the appropriateness of each member from the current population is determined using a cost function which is explained later and the optimal members are selected as the next generation population. New chromosomes are created from the previous chromosomes using the crossover and the mutation procedures. In this study, three methods of single point crossover, double point crossover, and uniform crossover are used and a rate of 0.08 is selected for the mutation procedure using a swap method with a rate of 0.3, and the mutation probability for each gene is considered as 0.02. For the selection, the Roulette Wheel method with a selection pressure of 8 is used. This routine is iterated until it reaches the intended number of iterations and the optimal amount for the cost function.
Figure 1. Study workflow and feature selection
4. Cost Function
The cost function used in the genetic algorithm of this study is a double target cost function in which by reducing the amount of MMRE, the minimal number of possible features is selected from the dataset. In the following equation, β is an independent positive number indicating the cost of adding a feature with which we can create balance between the two scales nf (number of selected features) and MMRE (mean estimation error rate for all target samples).
(5)
In order to calculate the MMRE, a hybrid linear regression and a decision tree has been used. Regression analysis is a statistical method for estimating the relation between variables creating an opportunity for predicting the impact of one variable on multiple variables and giving a better understanding of a variable change status at the change time of each independent variable. The regression model calculates Y as a function of X and β in which β is the unknown parameter, X is the independent variable, and Y is the dependent variable [8].
(6)
The decision tree is a flow chart with a tree structure in which each inner node performs a test on one of hybrid features. Each branch shows an output of the test and each leaf node indicates the label or estimated amount for that sample. The strongest node of a tree is the root [14]. In this study, we made use of classification and regression tree (cart) algorithms.
5. Datasets
The datasets used in this study are Maxwell, Cocomo81, and Desharnais. The reason for choosing these three datasets is that these sets contain relatively new data on a large number of software projects from the largest banks in the world are perhaps some of the most common datasets used in effort estimation studies. Thus, these datasets were chosen in order to compare the concluding results with other studies [5-15-16]. The datasets will be explained in further detail.
The Maxwell dataset includes relatively new data in 62 fields of software projects from the largest world banks in Finland in which each project describes 26 features. This data set has 25 independent variables with various software features including application type, size, etc. All of these features with the exception of the first, 24th, 25th, and 26th features are classified. These features are numerical. The independent variable of the determined efforts along with the work hours accomplished by the software exhibitor is technical specifications until delivery [5, 15]. Statistical information related to the Maxwell dataset is presented in table 1.
The Cocomo81 dataset includes 63 software projects such as commercial, scientific, systematic, on-time, and support projects. It has 16 independent variables which are measured using product, project, computer, and personal attributes. The dependent variable is the software development effort measured using each individual’s hours [15]. Statistical information related to the Cocomo81 dataset are presented in table 2.
The Desharnais dataset is one of the most common datasets in effort estimation. This dataset includes 81 software project samples in which four samples include faulty data; thus, only 77 samples have been considered. In this dataset, each sample is described using 11 features, 10 of which are independent, and only one feature is dependent. In this study, the dependent effort variable estimation has been used using each individual’s hours [16]. Explanations and information about the Desharnais dataset variables are presented in table 3.
Table 1
Maxwell dataset features
Feature | Description | Mean | Std Dev | Min | Max |
Time | Time | 5.58 | 2.13 | 1 | 9 |
App | Application type | 2.35 | 0.99 | 1 | 5 |
Har | Hardware platform | 2.61 | 1 | 1 | 5 |
Dba | Database | 1.03 | 0.44 | 0 | 4 |
Ifc | User interface | 1.94 | 0.25 | 1 | 2 |
Source | Where developed | 1.87 | 0.34 | 1 | 2 |
Telonuse | Telon use | 2.55 | 1.02 | 1 | 4 |
Nlan | Number of different development languages used |
0.24 |
0.43 |
0 |
1 |
T01 | Customer participation | 3.05 | 1 | 1 | 5 |
T02 | Development environment adequacy | 3.05 | 0.71 | 1 | 5 |
T03 | Staff availability | 3.03 | 0.89 | 2 | 5 |
T04 | Standards use | 3.19 | 0.70 | 2 | 5 |
T05 | Methods use | 3.05 | 0.71 | 1 | 5 |
T06 | Tools use | 2.90 | 0.69 | 1 | 4 |
T07 | Software’s logical complexity | 3.24 | 0.90 | 1 | 5 |
T08 | Requirements volatility | 3.81 | 0.96 | 2 | 5 |
T09 | Quality requirements | 4.06 | 0.74 | 2 | 5 |
T10 | Efficiency requirements | 3.61 | 0.89 | 2 | 5 |
T11 | Installation requirements | 3.42 | 0.98 | 2 | 5 |
T12 | Staff analysis skills | 3.82 | 0.69 | 2 | 5 |
T13 | Staff application knowledge | 3.06 | 0.96 | 1 | 5 |
T14 | Staff tool skills | 3.26 | 1.01 | 1 | 5 |
T15 | Staff team skills | 3.34 | 0.75 | 1 | 5 |
Duration | Duration | 17.21 | 10.65 | 4 | 54 |
Size | Application size | 673.31 | 784.08 | 48 | 3,643 |
Effort | Effort | 8223.21 | 10,499.90 | 583 | 63,69 |
Table 2
COCOMO81 dataset features.
Feature | Description | Mean | Min | Max |
Rely | Reliability
| 1.036 | 0.75 | 1.4 |
Data | Data size | 1.004 | 0.94 | 1.16 |
Cplx | Complexity
| 1.091 | 0.70 | 1.65 |
Time | Execution time constraint | 1.114 | 1 | 1.66 |
Store | Main storage constraint
| 1.008 | 1 | 1.56 |
Virt | Virtual machine volatility | 0.972 | 0.87 | 1.3 |
Turn | Computer turnaround time | 0.905 | 0.87 | 1.15 |
Acap | Analyst capability | 0.949 | 0.71 | 1.46 |
Aexp
| Application experience
| 0.949 | 0.82 | 1.29 |
Pcap | programmer capability | 0.937 | 0.70 | 1.42 |
Vexp | Virtual machine experience | 1.005 | 0.90 | 1.21 |
Lexp | Programming language experience | 1.001 | 0.95 | 1.14 |
Modp | Modern programming practice | 1.004 | 0.82 | 1.24 |
Tools | Use of software tools | 1.017 | 0.83 | 1.24 |
Sced | Required development schedule | 1.049 | 1 | 1.23 |
Loc |
| 77.209 | 1.98 | 1150 |
actual |
| 683.3206 | 5.9 | 11400 |
IV. results
In this section, the outcomes of performing this study according to the method proposed in previous sections will be examined. The proposed method is executed and evaluated on MATLAB software using the three datasets Maxwell, COCOMO81, and Desharnais. As mentioned before, feature selection is done using a binary genetic algorithm, and for examining the evaluation criterion, a hybrid linear regression method and a decision tree along with a tree regression algorithm has been used. Results of hybrid linear regression analysis before and after feature selection are presented in figure 2 which shows a reduction in parameters of MMRE, MdMRE, and STD and an increase in PRED for the three given datasets after applying genetic algorithm and feature selection. 8, 13, and 16 features have been selected from the Desharnais dataset with 10 features, COCOMO81 dataset with 16 features, and Maxwell data with 25 features, respectively.
Table 3
Desharnais dataset features
Feature | Description | Type | Statement |
TeamExp | The team experience measured in years | Numerical | Measured in years |
ManagerExp | The manager experience measured in years | Numerical | Measured in years |
YearEnd | The project year finished | Categorical | Determined by year |
Length | The length of the code (dependent variable) | Numerical | Measured in Month |
Transactions | The number of basic logical transactions in the system model | Numerical | Number of transactions |
Entities | The number of the entities in the system data model | Numerical | Number of entities |
PointsNonAdjust | The Non Adjusted function points | Numerical | Number of Non adjusted function ponts |
AdjustFctor | Sum of complexity factors | Numerical | Sum of complexity factors |
PointsAdjust | The Adjusted function points | Numerical | Number of adjusted function points |
Language | The language used to develop the system | Categorical
| 1 = 1st generation 2 = 2nd generation 3 = 3rd generation |
Effort | The Development effort (dependent variable) | Numerical | Measured in Person-Hours |
Figure 2. results of hybrid linear regression analysis before and after feature selection
Results of decision tree algorithm using tree regression analysis before and after feature selection are shown figure 3. Like the previous analysis, three data sets with 8, 13, and 16 features have been selected, respectively. As indicated by figure 3, the amount of operational parameters MMRE, MdMRE, and STD after applying genetic algorithm and feature selection have decreased, and the amount of PRED has increased in comparison to the initial state where all features were utilized.
Figure 3. results of decision tree (cart) analysis before and after feature selection
Finally, the features selected from the three data sets are listed in table 4. 3, 3, and 9 features were selected from the Desharnais, COCOMO81, and Maxwell datasets, respectively. Thus, with fewer features, we have been able to estimate more accurately the effort needed for software development.
Table 4
selected features from datasets
Feature selection (GA) | Data set | |
selected features | number of features | |
1, 4, 5, 6, 7, 8, 10, 11 | 8 | Desharnais |
1, 2, 3, 4, 5, 6, 7, 9, 10, 11, 14, 15, 16 | 13 | Cocomo 81 |
1, 4, 6, 7, 8, 9, 10, 11, 12, 13, 17, 18, 20, 22, 23, 24 | 16 | Maxwell |
V. conclusion
The proposed method of this article was implemented and tested on the Maxwell, the COCOMO81, and the Desharnais datasets indicating that feature selection can be effective in increasing effort estimation accuracy.
According to the presented results in the previous section and considering the fact that lower MMRE mean results in lower error rate and higher accuracy of effort estimation, it can be said that in each of the three datasets, applying feature selection along with a binary genetic algorithm in both the hybrid linear regression and the decision tree CART, we were able to reach lower error rates and higher accuracy of effort estimation. In all three datasets, Desharnais, Maxwell, and CCOMO81 which had 11, 25, and 16 features, respectively, by only applying 8, 6, and 13 features selected by the genetic algorithm, we were able to not only estimate the effort with an equal accuracy, but with even higher accuracy and lower errors in comparison to the previous situation. This fact indicates the positive effect of feature selection in improving effort estimation accuracy.
Thus, the results indicate that better outcomes can be achieved in regards to increasing effort estimation accuracy and reducing error rates in different software projects using fewer features and smaller feature datasets which can decrease the complexity of the model and increase accuracy which ultimately reduces time loss during computations.
References
[1] Y. S. Seo, et al, “AREION: Software effort estimation based on multiple regressions with adaptive recursive data partitioning”, ELSEVIER, Information and Software Technology, vol.55, pp. 1710-7725, 2013.
[2] A. S. Grewal, et al, “Emerging Estimation Techniques”, International Journal of Computer Applications (0975 – 8887), vol. 52, no. 8, pp. 30–34, 2012.
[3] Hatami, Nafiseh, “Examination of Feature Selection Based Methods”, ict center, Malek-Ashtar University of Technology, 2013.
[4] E. Papatheocharous, et al, “Feature Subset Selection for Software Cost Modelling and Estimation”, 2010.
[5] V. Khatibi, et al, “Increasing the Accuracy of Analogy Based Software
Development Effort Estimation Using Neural Networks”, International Journal of Computer and Communication Engineering, Vol. 2, No. 1, pp. 78-81, 2013.
[6] A. Zaid, et al, “Issues in Software Cost Estimation,” International Journal of Computer Science and Network Security, vol. 8, no. 11, pp. 350–356, 2008.
[7] F. Ferrucci, et al, “Genetic Programming for Effort Estimation:
An Analysis of the Impact of Different Fitness Functions”, 2nd International Symposium on Search Based Software Engineering, PP. 89-91, IEEE, 2010.
[8] M. Singh, et al, “Software Productivity Empirical Model for Early
Estimation of Development”, International Journal of Computer Science and Information Technologies, Vol. 5 (1), 2014, pp. 682-685, 2014.
[9] M. Azzeh, et al, “An Optimized Analogy-Based Project Effort Estimation”, International Journal of Advanced Computer Science and Applications, Vol.5, no.4, pp. 6-12, 2014.
[10] R. p, et al, “Building Software Cost Estimation Models using Homogenous Data”, IEEE, First International Symposium on Empirical Software Engineering and Measurement, PP.393-400, 2007.
[11] M. Karagiannopoulos, et al. “Feature Selection for Regression Problems”,
Educational Software Development Laboratory, Department of Mathematics, University of Patras, Greece, 2004.
[12] H. Hamza, et al, “Software Effort Estimation using Artificial Neural Networks: A Survey of the Current Practices”, IEEE, 10th International Conference on Information Technology: New Generations, PP.731-733, 2013.
[13] M. Melanie, an Introduction to Genetic Algorithms, Cambridge, Massachusetts. London, England, Fifth printing, 1999.
[14] J. Han and M. Kamber, Data Mining:Concepts and Techniques, Second Edition, Elsevier, University of Illinois at Urbana-Champaign, 2006.
[15] M. O. Elish, et al, “Empirical Study of Homogeneous and Heterogeneous Ensemble Models for Software Development Effort Estimation”, Hindawi Publishing Corporation Mathematical Problems in Engineering,Vol 2013, Article ID 312067, 21 pages, http://dx.doi.org/10.1155/2013/312067, 2013.
[16] V. Khatibi, et al, “A New Fuzzy Clustering Based Method to Increase the Accuracy of Software Development Effort Estimation”, World Applied Sciences Journal, vol.14 (9), pp.265-1275, 2011.
