A device for measuring the ability of a human operator to stabilize an inherently unstable system so as to maintain an output signal on a display terminal within a preset limit. The difficulty is automatically increased at a relatively fast rate until the human operator approaches the limits of his ability to maintain the output signal within the limits. The difficulty is then increased at a relatively slow rate until the human operator is no longer able to maintain the output of the testing device within preselected limits. The device is mechanized using a general purpose microprocessor and utilizes binary shifts rather than multiplication or division operations in order to achieve real time operation at a sufficiently fast rate in order to test the limits of human ability. The device also provides for the measurement of the ability of the human operator after an adaptation or training period by maintaining the instability of the system at a predetermined level while monitoring and measuring the number of times that the human operator loses control of the output signal during a predetermined period of testing and by measuring and monitoring the period of time that elapses before the first loss of such control. The computational procedure utilized in the device includes provisions for avoiding a flickering effect that otherwise would appear on the display terminal.