Apparatus and method for displaying changes in statistical parameters in a process control system

At least one statistical output associated with a process model and a rate of change associated with each statistical output is identified, such as by using historical data associated with the process model. The statistical outputs and the rates of change are used to generate a graphical display, such as a phase plane plot. Each point in the display is based on one of the statistical outputs and its associated rate of change. The graphical display could include multiple portions, such as quadrants, and one of the portions can be selected and highlighted. The different portions of the graphical display may represent whether the process model is a poor fit to current conditions and approaching a better fit, a poor fit and approaching a poorer fit, a good fit and approaching a poorer fit, and a good fit and approaching a better fit.