Table 1.

Abbreviations of models.

Number	Abbreviation	Definition
1.	GM(1, 1)	Grey model with one variable and one first order equation [31]
2.	Verhulst	Verhulst grey model [32]
3.	ARGM(1,1)	Autoregressive grey model [33]
4.	ONGM(1,1)	Optimized NGM(1,1,k,c) model [34]
5.	ENGM(1,1)	Exact nonhomogeneous grey model [35]
6.	ARIMA	Autoregressive Integrated Moving Average model [36]
7.	NGBM(1,1)	Nonlinear Grey Bernoulli Model [37]
8.	GRM(1,1)	Grey Richards model
9.	GERM(1,1,$e^{at}$)	Grey Extend Richards model