Comparison between the Conventional Partial Least Squares (Pls) and the Robust Partial Least Squares (Rpls-Sem) Through Winsorization Approach

Introduction

PLS-SEM was seen to be the technique which could be applied if the predictor variables displayed high or perfect multicollinearity (Hair et al., 2017). On the other hand, robust methods were developed for decreasing or eliminating the effects of all outliers (Maronna and Zamar, 2002). . In this study, the researchers proposed a novel RPLS-SEM model which was based on the robustification of a covariance matrix that was used in the classical PLS algorithm. This study also chose a robust covariance estimator, which used the Winsorisation estimator for estimating the covariance matrix in the multivariate dataset for decreasing the harmful effect of the outliers. Croux and Rousseeuw (1992) stated that a robust estimator (or a Winsorised estimator, W) could be used instead of the popular mean vector, which could substitute the inverse of the Winsorised covariance matrix. This technique was called the Robust Straightforward Implementation of the statistically- inspired Modification of PLS (RSIMPLS). Thereafter, the researcher compared the novel and the classical PLS-SEM models.

Data

In this study, the researchers collected the secondary data from the Al-Zawiya Steam Power plant in Libya. Real data related to power generation was collected and compiled by the Technical Department of the AL-Zawiya Oil Refining Company the important input parameters for freshwater and power generation, which included:

Desalination unit (DW), i.e., the amount of steam (tons/day) and seawater (m3/ day) needed for freshwater production.

Steam Power Plant (SPP) requirements - steam turbine (tons/day) and boiler (m3/ day of distilled water).

Chemical Additives (CA) - Phosphate (kg/day), Morphine, anti-scale and hydrazine (L/day).

Maintenance and Operation (OP) – mean costs for the chemical treatment and fuel (LYD/day).

Figure 1 presents an arrow diagram, wherein the researcher assumed that every MV (measured variable) block could be summarized by an LV (unmeasured). The following endogenous LV symbols were suggested: DW is desalination units represent steam (D1) and seawater (D2) ; Steam power plant SPP represent steam turbines (S1) and boiler (S2) ; while CA represents chemical additive consists of four indicator variables are quantity of sodium triphosphate,(C1), hydrazine (C2), morphine (C3) and anti-scale (C4) needed; whereas the exogenous latent variables were represented as OP includes chemical treatment (O1) and fuel-related costs (O2); and Output is electricity (P1) and fresh water supply (P2) . The general structural and measurement models for DW, SPP, CA, OP and Output were as explained in figure1.

Data Analysis

The researchers used a SmartPLS3 software (Ringle et al., 2015) as it offers appropriate techniques for facilitating the fitting of the specific model. This software generated the data processing output, which included the general model fit statistics and all parameter estimates, described in Figure 1 and Figure 2. The causality model presented in this figure summarized the steps involved in a structural regression of an RPLS-SEM model.

The quality of the PLS-SEM model was assessed using two steps: initially, the measurement model was assessed and if it satisfied all criteria, the structural model was evaluated. The measurement model was investigated using parameters like Cronbach’s alpha, composite reliability and Average variance extracted (AVE). Tables 1 and 2 presented the RPLS-SEM and PLS-SEM model indices. The results indicated that the Cronbach’s alpha values for both the models were greater than 0.7, which showed the indicator homogeneity. Furthermore, the cut-off values for the composite reliability were larger than 0.8, while the AVE was greater than 0.5, which indicated that more than 50% of the variance of the indicators could be explained (Chin, 2010).

Figure 1. Partial Least Square-Path Modelling

Figure 2. Robust Partial Least Square-Path Modelling

Table 1. Reliability Assessment for the RPLS-SEM

Table 2. Reliability Assessment of the PLS-SEM

All the indices for the PLS-SEM were higher due to the presence of the internal consistency, based on the average correlation amongst the items (multicollinearity).

Secondly, the inner model quality was assessed by investigating the indices of the coefficient of determination, bootstrapping, redundancy index, and the Goodness of Fit (GoF) index. The structural model assessment includes the testing of the relationships between all model constructs shown in Tables 3 and 4. The RPLS-SEM model showed no significant fluctuations, which showed that the RPLS-SEM was better than the PLS-SEM model. Esposito Vinzi et al. (2010) stated that the assessment of the non-significant path coefficients should be carried out carefully, due to the presence of multicollinearity. Finally, the PLS-SEM model showed a higher coefficient of determination, redundancy index, and GoF values since these indices were based on the correlation (multicollinearity issue).

Table 3 and Table 4 present the results of the bootstrapping technique conducted on the different resampled datasets. The significant fluctuations noted in the results were based on the differing number of resampling data groups, except in 500 re- sampled data sets, where the RPLS-SEM model showed a good performance.

Table 3. Structural PLS-SEM Model Analysed Using the Bootstrap Process

* indicates the significance at 0.05 level of significance.

** indicates the significance at 0.01 level.

Table 4. RPLS-SEM Structural Model Assessment Using the Bootstrap Process

* Significance at 0.05 level ** significance at 0.01 level

The data showed that multicollinearity existed in the PLS-SEM model (Table 5); whereas the variance inflation factors (VIF) values in the RPLS-SEM were seen to be less than 5 (Table 6). Hence, the researcher proposed the RPLS-SEM for overcoming the multicollinearity in the study.

Table 5. VIF Values for the Outer PLS-SEM Model

Table 6. VIF Values for the Outer RPLS-SEM Model

The results compared the performances of the PLS-SEM and the RPLS-SEM and showed that the RPLS-SEM was more effective than the PLS-SEM model in overcoming the multicollinearity problem.

Conclusion

The results and the analysis of the data set derived from the Libyan Oil Refining sector showed that the novel RPLS-SEM model was very effective and robust. This model showed a higher efficiency and displayed a better predictive capacity compared to the conventional PLS-SEM model. Finally, it was stated that this robust model was able to efficiently cope with the data set and provide robust predictions.

Conflict of interest

The authors declare no potential conflict of interest regarding the publication of this work. In addition, the ethical issues including plagiarism, informed consent, misconduct, data fabrication and, or falsification, double publication and, or submission, and redundancy have been completely witnessed by the authors.

Funding

The author (s) received no financial support for the research, authorship, and /or publication of this article