Multivariate Process Monitoring
Multivariate Process Monitoring Or, put differently, the SD of a sample is the distance to the center, measured in the hyperplane of the PCA projection, and the OD is the distance to this hyperplane. Obviously, both SD and OD depend on the number of PCs. When a sample is above the horizontal threshold it is too far away from the PCA subspace; when it is to the right from the vertical threshold it is too far from the other samples within the PCA subspace. The horizontal and vertical thresholds are derived from χ2 approximations (Todorov and Filzmoser 2009). For the Grignolino data, this leads to the plot in Fig. 11.6: Several of the most outlying samples are indicated with their indices, so that they can be inspected further. Also a biplot method is available, which shows a plot that is very similar to the right plot in Fig. 11.5. Inspection of the data shows that objects 63 and 15 do contain some extreme values in some of the variables—indeed, object 63 is also the object w