A diagnostic device and method for monitoring the operation of a slave or ratio control loop in a meshed control structure of an automation system. The diagnostic device includes an evaluation device and a data memory for storing sequences of setpoint data and actual value data. The evaluation device determines a first dimension for the scatter of the actual-value data and a second dimension for the scatter of the setpoint data. A characteristic number (CPIVar, CPIKas) for evaluating control quality is determined and/or displayed as a function of the ratio of the first dimension to the second dimension to enable an operator to evaluate the control loop status, permitting automated control loop evaluation of a fluctuating setpoint.