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Bootstrap Aggregating Multivariate Adaptive Regression Splines (Bagging MARS) to Analyse the Lecturer Research Performance in Private University

This study explores the use of a fairly simple nonparametric regression algorithm known as multivariate adaptive regression splines (MARS) which has the ability to approximate the relationship between the predictor and response variables. The main advantages of MARS are its capacity to produce simple, easy-to-interpret models, its ability to estimate the contributions of the predictor variables, and its computational efficiency. In this research, MARS is combined with one of nonparametric approach bootstrap aggregating (bagging). Bagging is used to improve the classification accuracy of the MARS method. This study is aimed to analyse lecturer research performance in private university using Bagging MARS algorithm. In modelling bagging MARS for lecturer research performance in a private university that there are three dominant influencing factors: 1) amount of research with cost an internal college granted by internal college with interest level of 86.20%; 2) number of publications of research results in national journals with interest level of 69.83%; and 3) number of speakers within national scientific meeting/ seminar with interest level of 63.34%. The accuracy of the MARS model classification is 84.615% with the classification error rate (APER) of 15.385%.


Introduction
Multivariate Adaptive Regression Splines (MARS) is one of nonparametric and semi parametric regression that takes into accounts the covariates with a multivariate approach [1]. Nonparametric regression using MARS method does not depend on the assumption of a certain curve shape, has flexibility in high dimensional data and modelling involves a lot of interaction with a few variables [2]. MARS is a hybrid of nonparametric methods, like recursive partitioning regression and additive model. The data are left to reveal the variable knot locations, while the users need not input any specification into the model [3]. The basis functions in MARS (which serve as independent variables) are truncated linear functions, which address the problem of discontinuity of recursive partitioning algorithms. In contrast to additive models, MARS allows interactions up to an order specified by the user and trades off the interaction order and complexity of the additive functions and interactions [4]. Certainly, MARS is a highly commended predictive modelling technique in almost all applications [5]. Currently MARS is often combined with a nonparametric approach to modelling flexibility. In this research, MARS is combined with one of nonparametric approach bootstrap aggregating (bagging). Bagging method is used as a tool to form a more stable classifier. Bagging predictor is a method to generate multiple versions of predictors and use them for aggregate predictors [6]. In this case, bagging is used to improve the classification accuracy of the MARS method. Therefore, this study is expected to obtain better modelling and classification functions through bagging MARS method.
The main objective of this research is to analyse lecturer research performance in a private university (case study at STKIP PGRI Tulungagung) using Bagging MARS method. Research performance is one component in the determination of the lecturer performance index or Indeks Kinerja Dosen (IKD). In a previous study by [7], IKD is modelled by using Survival MARS, where the response variables studied were time intervals. Lecturers are professional educators and scientists with the main task of transforming, developing and disseminating science, technology and the arts through education, research and dedication to society [8,9]. Lecturer performance is defined as the ability to carry out the work or tasks that lecturers have in completing a job [10]. Performance can be interpreted as work presentation, job performance, work achievement, or work result [11]. It also refers to the result or output of a process. Education is aimed to: (1) improve the performance, capability and output of education; (2) facilitate communication and information exchange about the best practice of education with various types of educational institutions; and (3) be a tool to improve institutional performance education and guidelines in strategic planning.
Through the use of the MARS Bagging approach, it is expected to obtain the dominant factors that influence the lecturer's research performance more accurately. The classification produced by the MARS bagging model is expected to be optimal. The results of this study can be used as input for the leadership of private universities for policy making improvement of lecturer performance quality.

Multivariate Adaptive Regression Splines (MARS)
In 1991, Jerome H. Friedman introduced the Multivariate Adaptive Regression Spline (MARS) method as a new method that automates the development of accurate predictive models for continuous and binary response variables. MARS is one of the flexible methods for modelling high-dimensional regression data. MARS is a form of extension of the Basis Spline Functions where the number of basis function is the parameters of the model [4]. Some terms that need to be considered in MARS is as follows [5], a. Knots, the point of a regression line to form a region of a regression function. b. Basis Function (BF), collection of some functions that are used to describe the relationship between the response variable and the predictor. c. Interaction, a correlation between variables and the maximum number of interaction (MI) 1, 2, and 3.
MARS model is formulated as follows: where, The best model produces smallest GCV which is denoted as: where,

Bootstrap Aggregating (Bagging)
The first bagging method used by [18] as a tool to form a more stable classifier. Bagging predictor is a method to generate multiple versions of predictors and use them for aggregate predictors. Multiple versions are formed by a bootstrap replication of a data set. In some cases, bagging on real data sets can improve accuracy. If the changes in the data sets lead to significant changes then bagging can improve accuracy. The basic idea of bagging is to use bootstrap resampling to generate predictors with many versions, which when the combination should be better than a single predictor built to solve the same problem. Determination of the amount of replication B is very varied, because the size of B can give different results at each stage of analysis. Based on the References [13] it recommends 25 or fifty times replications, but [14] suggests that increased accuracy will occur if the number of replications is increased from fifty to one hundred times, whereas if the number is increased to more than a hundred times it will produce no greater accuracy of the replication accuracy of a hundred times. Meanwhile, References [15] recommend small values of B, such as 25 times.

Research Methodology
This research used a quantitative approach because the process of this research is deductive. Response variable of this study is lecturer research performance, that measured for both lecturer as civil servant (DPK) and lecturer with permanent status (DTY). Categorical response variable has goal to be suitable with MARS binary response. The predictor variable in this study is a research activity consisting of eleven indicators. The sample used are 35 lecturers at STKIP PGRI Tulungagunng. Technique of collecting data in this research is survey with questionnaire instrument.
Steps of analyses in this study are as follows:

Analysis and Discussion
Determining the best MARS model is based on the minimum GCV value obtained by trial and error in combining BF, MI, and MO criteria to get the best model. The MARS modelling obtained from the best combination of BF, MI and MO was selected based on minimum GCV values. When GCV is the same, the next consideration is to choose the model with the largest R2 value, if it turns out that some of the models have the same R 2 , then the consideration switches to the greatest classification (total correct). From the overall MARS model that has been obtained based on the minimum GCV value, the best model of MARS selected is the model with the value of BF = 66, MI = 3, and MO = 2 and the GCV value of 0.018; the value of R 2 is 93.2%; and the value of classification accuracy (total correct) of 88.524%. The best MARS model is: The best MARS model in Equation (4) is shaped by seven variables that significantly contribute to the model. The importance of each variable to the model is shown in Table 2.  Table 2 shows the importance of predictor variables in the grouping function, which is estimated by the increase of GCV values due to the removal of the variables considered from the model. In The next step is to evaluate the classification accuracy of the MARS model. Evaluate the classification of private high school quality by calculating the value of APER obtained through the following classification Table 3.
Based on the above calculation, it can be seen that the accuracy of MARS model classification is 82.857% with the classification error rate (APER) of 17.142%. Furthermore, a bagging approach is used to find out the accuracy of the classification of the MARS model. Bagging with thirty replications is proved to demonstrate the accuracy of MARS model classification with single datasets, comparison of classification results for each {£ ( ) } with the result of MARS model classification of single dataset along with the decrease of classification error level shown in Table 4.

Conclusion
Based on the empirical results using Bagging MARS approach, the best model of MARS of lecturer research performance selected is the model with the value of BF = 66, MI = 3, and MO = 2 and the GCV value of 0.018; the value of R 2 is 93.2%; and the value of classification accuracy (total correct) of 88.524%. The obtained significant influencing predictors of the lecturer research performance are 1) amount of research with cost an internal college grant with interest level of 86.20%; 2) number of publications of research results in national journals with interest level of 69.83%; and 3) number of speakers within national scientific meeting/ seminar with interest level of 63.34%. The classification accuracy of MARS model is 82.857% with the classification error rate (APER) of 17.142%.