A diagnostic device and method for monitoring operation of automation system control loops includes an evaluation device and a data memory storing sequences of actual-value data of the control loops. An absolute value maximum of a cross-correlation function, for an excitation resulting from changes in a setpoint setting of one control loop, is determined as a first quantitative measure for positive time lag of the sequence of actual-value data of another control loop, and a numeric parameter evaluating an interaction effect of the one control loop on the other is determined as a function of the first quantitative measure. Strength and direction of the interaction effect are displayed. The calculations are repeated for all pairs of monitored control loops and a matrix of the numeric parameters is displayed.