A method of visually reproducing actual and criterion value signals without giving any numerical values includes signal processing of the measuring values forming the basis of the actual value signal, actual and criterion values being fed to a measuring instrument, there to operate at least one indicator (8, 9), the sweep of which defines a surface (S) constituting a measure of the discrepancy between the actual and criterion value signals. The indicator may be of the analogue or quasi-analogue type. Coincidence between actual and criterion signals is indicated by a line, i.e. the surface is reduced to zero.
