This paper considers a semiparametric generalized autoregressive conditional heteroscedastic (S-GARCH) model. For this model, we first estimate the time-varying long run component by the kernel estimator, and then estimate the non-time-varying parameters in short run component by the quasi maximum likelihood estimator (QMLE). We show that the QMLE is asymptotically normal with the parametric convergence rate. Next, we provide a consistent Bayesian information criterion for order selection. Furthermore, we construct a Lagrange multiplier test for linear parameter constraint and a portmanteau test for model checking, and obtain their asymptotic null distributions. Our entire statistical inference procedure works for the non-stationary data with two important features: first, our QMLE and two tests are adaptive to the unknown form of the long run component; second, our QMLE and two tests share the same efficiency and testing power as those in variance target method when the S-GARCH model is stationary. Engle (1982) and Bollerslev (1986), the generalized autoregressive conditional heteroscedastic (GARCH) model is perhaps the most influential one to capture and forecast the volatility of economic and financial return data.
Introduction. Since the seminal work ofHowever, the GARCH model is often used under the stationarity assumption. Due to