1. A process control system for detecting abnormal events in a process having one or more independent variables and one or more dependent variables, which system comprises means for measuring values of the one or more independent and dependent variables, a process control means comprising a predictive model for calculating predicted values of the one or more dependent variables from the measured values of the one or more independent variables, a means of calculating residual values for the one or more dependent variables from the difference between the predicted and measured values of the one or more dependent variables, and a means for performing a principal component analysis on the residual values, characterised in that the process control means is a multivariable predictive control means.