The invention relates to a method for comparing variable values obtained from different versions of an application program in an automated system. An input signal obtained from an automated process is inputted to at least two different versions of the application program residing within a same controller. The same tasks are executed based on the input signal for the different versions, and process related variable values obtained from the execution of the versions are stored in memory locations known by the controller. Variable values from the different versions and derived from corresponding execution steps of the versions are retrieved from the memory locations, and compared during an idle time. An improved way of revising an automation system is thereby achieved. The invention also relates to such controller and an automation system.