My question is: how to specify an ARMA model (p=?, q=?) based on the above figure? The author did give some ideas, but I'm totally confused. The first eight lags of PACFs are significant. This implies that q (order of MA part) >0. Why? The PACFs do not follow a simple exponentially decaying pattern. This means p (order of AR part) >1. Why? Somehow I think AR(8) would be a good choice. Is it reasonable?
Is there any general rule about how to specify an ARMA model? Thanks!