“…The T 2 statistic measures the variation of principal components, and the Q statistic measures the variation of nonprincipal components. Later on, many improvements have been studied, such as the multiway PCA for batch processes (Nomikos and MacGregor, 1994), dynamic PCA introducing dynamic behavior into the PCA model (Ku et al, 1995), multiscale PCA based on wavelet analysis (Bakshi, 1998), recursive PCA (Li et al, 2000), dynamic PCA for batch monitoring with time-lagged windows (Chen and Liu, 2002), kernel PCA for nonlinear process monitoring (Cho et al, 2005;Lee et al, 2004;Schölkopf et al, 1998), and Robust multiscale PCA (Wang and Romagnoli, 2005). Qin (2003) reviewed several fault detection indices associated with T 2 statistic and Q statistic and compared the reconstruction-based approach and the contribution-based approach with simulation and industrial examples.…”