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 for 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, wherein the process control means is a multivariable predictive control means and the principal component analysis results in the output of one or more scores values, T2 values and Q values, an abnormal event being identified from one or more thereof, and wherein the Q or T2 contributions of a data point associated with an abnormal event are compared to a database of Q or T2 contributions associated with known abnormal events in order to identify the onset of a known abnormal event.