“…The gray theory was proposed by Deng (Deng, 1982; Deng, 1989), which included gray relational analysis (GRA). GRA is an effective method to analyze uncertain correlations between things and system factors, or factors and the main behavior, and enable investigation of quantitative and qualitative relationships using relatively little data or with considerable variability in factors (Deng 1982; Deng, 1989; Wang et al, 2009; Liu and Lin, 2010; Zhang and Zhang, 2013; Guo and Kang, 2018). In addition, because there is no restriction on the type of samples and the probability distribution, GRA has been widely applied in diverse fields of research (Wang et al, 2009; Zhang and Zhang, 2013; Guo and Kang, 2018; Sun et al, 2018; Wang et al, 2018).…”