Abstract.—Performances in different man‐machine system functions are usually not directly comparable:It is suggested that some functions may be measured through quantities corresponding to β andd'of the signal Detection Theory. Necessary assumptions are that the task can be reduced to a binary decision and that the decision depends upon a subjective value of certainty which vanes in one dimension. The method of measurement is illustrated by an example with a hypothetical system comprising three functions: detecting, identifying and interpre