1. A method of monitoring a processing system for processing a substrate during the course of semiconductor manufacturing, comprising: acquiring data from said processing system for a plurality of observations, said data comprising a plurality of data parameters; constructing a principal components analysis (PCA) model from said data, including centering coefficients; acquiring additional data from said processing system, said additional data comprising an additional observation of said plurality of data parameters; adjusting said centering coefficients to produce updated adaptive centering coefficients for each of said data parameters in said PCA model; applying said updated adaptive centering coefficients to each of said data parameters in said PCA model; determining at least one statistical quantity from said additional data using said PCA model; setting a control limit for said at least one statistical quantity; and comparing said at least one statistical quantity to said control limit.