“…There are fundamentally two types of joint schemes for and . On the one hand, the schemes make use of a single control chart to monitor both parameters; see, for example, Spiring and Cheng, 20 Chen et al, 21 Khoo, 22 Chen and Thaga, 23 Yeh and Lin, 24 Zhang et al 25 On the other hand, the most popular joint schemes that result from running simultaneously two individual charts, one for and another one for , such as the ones discussed by Chen and Thaga, 23 Reynolds and Cho, 26 Hawkins and Maboudou-Tchao, 19 Machado and Costa, 27 Reynolds and Stoumbos, 28 Zhang and Chang, 29 Costa and Machado, 30 Reynolds and Cho, 31 Ramos et al, 32,33 Ramos,34,35 Ramos et al, 36 Morais et al 6 When we use any of these joint schemes, the multivariate quality characteristic is deemed to be out of control whenever a signal is triggered by either individual chart. Thus, a shift in the mean vector can be misinterpreted as a shift in the covariance matrix and vice versa.…”