Model of Automobile Parts Sale Prediction Based on Nonlinear Periodic Gray GM(1,1) and Empirical Research

Table 1

Model fitting accuracy class.

Accuracy class	Index critical value
	Relative error (%)	Correlation degree	Mean variance ratio	Small error probability
The first grade (good)	1	0.90	0.35	0.95
The second grade (qualified)	5	0.80	0.50	0.80
The third grade (possibly qualified)	10	0.70	0.65	0.70
The fourth grade (unqualified)	20	0.60	0.80	0.60