3.1.2. Specification for marginal distribution
Because both growth rates of production and price in agriculture have the characteristics of heteroscedasticity, volatility
and skewness, etc., we use the ARMA-GARCH model in which standardized residuals satisfy the skewed-t distribution.
Bollerslev [37] proposed the GARCH (generalized autoregressive conditional heteroskedasticity) model that has replaced
the ARCH model in application; the GARCH model has since been widely used in econometrics, economics, etc. Following
Shiqing Ling [19], the ARMA (p, q)-GARCH (k, l) model can be formulated as: