Each curve plots the ratio of the best technical performance at a given point in time divided by the unaided

human power to accomplish the same function.

From CONCEPTUAL FOUNDATIONS FOR MULTIDISCIPINARY THINKING by Stephen Jay Kline

Stanford University Press 1995, p. 173.

(Reproduced from “Toward the Understanding of technology in society.” Mechanical Engineering (First of three articles – Mar, Apr, May, 1977.)