Multivariate control charts typically plot the statistic, which is a summary of multivariate variation. The classical statistic is defined in Classical Charts. When there is high correlation among the process variables, the correlation matrix is nearly singular. The subspace in which the process varies can be adequately explained by fewer variables than the original p variables. Thus, the principal component approach to multivariate control charts is to project the original p variables into a lowerdimensional subspace by using a model based on j principal components, where .
The key to the relationship between principal components and multivariate control charts is the decomposition of the sample covariance matrix, , into the form , where is a diagonal matrix (Jackson, 1991; Mardia, Kent, and Bibby, 1979). This is also the eigenvalue decomposition of , where the columns of are the eigenvectors and the diagonal elements of are the eigenvalues.
The statistic that is produced by the full principal component model is equivalent to the classical statistic. This is seen in the matrix representation of the statistic computed from a principal component model that uses all p components,

Because is the zero matrix by construction, then

Because , then

which is the classical form. Consequently the classical statistic can be expressed as a sum of squares,

where is the variance of the kth principal component.
Creating a chart that is based on a principal component model begins with choosing the number (j) of principal components. Effectively, this involves selecting a subspace in dimensions and then creating a statistic based on that jcomponent model.
The statistic is meant to monitor variation in the model space. However, if variation appears in the subspace orthogonal to model space, then the model assumptions and physical process should be reexamined. Variation outside the model space can be detected with an SPE chart.
In a model with j principal components, the statistic is calculated as

where is the principal component score for the kth principal component of the ith observation and is the standard deviation of .
The information in the remaining principal components is monitored with charts for the SPE statistic, which is calculated as
