An important consideration in the design of systems for human control is the difficulty of the task. An analytic measure of this subjective consideration is denned. The difficulty is a function of the lead factor which is an integrated measure of the amount of lead the human must produce. The theory together with experimental results imply that the performance function minimized by a human controller includes the amount of control as well as the amount of error. The difficulty is constrained by the capability, training and stress level of the operator. Remnant is accounted for by assuming that the operator generates additive noise as a function of the difficulty. The model is used to explain Bernotat's experimental results with prediction display.

This work was performed while the author was a guest professor at the Institut für Flugführung as part of the exchange programme between M.I.T. and the Technische Universität, Berlin.