Cross proposes human-behind-the-scenes prototypes, which would later be termed the wizard-of-Oz methodology:

There are two main problems which make investigation of the effects of new computer-aided design systems difficult. First, there is the probability that computer aids will so alter normal design processes that speculation based entirely on conventional practice cannot be reliable. Second, there is the prohibitive cost of setting up working experimental systems to provide the necessary experience and feedback.

[...]

The problem is, therefore, to devise a suitable simulation of a computer-aided design system. Current prototype systems consist essentially of a teleprinter console, usually plus a graphical device, through which the designer 'converses' with the computer—which may actually be some miles from the console. All that the user perceives of the system is this remote-access console, and the remainder is a black box to him.

Viewing the computer-aided design system in this way leads to an obvious suggestion for a simulation technique—one may as well fill the black box with people as with machinery. Doing so provides a comparatively cheap simulator, with the remarkable advantages of the human operator's flexibility, memory, and intelligence, and which can be reprogrammed to give a wide range of computer roles merely by changing the rules of operation. It sometimes lacks the real computer's speed and accuracy, but a team of experts working simultaneously can compensate to a sufficient degree to provide an acceptable simulation. This was the basic hypothesis of the research described here.