1. An apparatus for measuring the psychological condition of a patient comprising: means for receiving psychological test data relating to at least one patient and for presenting the received data as digital signals, said test data including data obtained with respect to a patient at various times during a time interval, said test data including data relating to at least one psychological variable; means for examining said digital signals and selecting therefrom the signal portions relating to said at least one psychological variable; means for computing a single-valued quantity for each of said various times, representing the psychological condition of a patient with respect to said at least one psychological variable, from said selected digital signals; and means for comparing said single-valued quantity computed for each of said various times with a predetermined benchmark quantity representing a benchmark psychological condition to obtain an indication of the patient's psychological condition with respect to said benchmark condition and an indication of the change in the patient's psychological condition over the time interval; wherein said comparing means includes means for comparing time changes in said single-valued quantity with a predetermined benchmark quantity representing a time rate of change of psychological condition, to obtain an indication of the time rate of change in the patient's psychological condition with respect to said benchmark rate.