In the field of human–computer interaction, a Wizard of Oz experiment is a research experiment in which subjects interact with a computer system that subjects believe to be autonomous, but that is actually operated or partially operated by an unseen human being.

The phrase Wizard of Oz (originally OZ Paradigm) has come into common usage in the fields of experimental psychology, human factors, ergonomics, linguistics, and usability engineering to describe a specific type of testing or iterative design. In such an experiment, a laboratory experimenter (the "wizard") simulates the behavior of a theoretical intelligent computer application, often by going into another room and intercepting all communications between participant and system. Sometimes this is done without the participant's prior knowledge, to manage the participant's expectations and encourage natural behaviors, while at other times the participant is aware.

For example, a test participant may think that he is communicating with a computer using a speech interface, when the wizard is actually covertly entering the participant's words into the computer, enabling them to be processed as a text stream, rather than as an audio stream. The missing system functionality that the wizard provides may be implemented in later versions of the system, or it may be speculative capabilities that current-day systems do not have; the precise details are generally considered irrelevant to the study. In testing situations, the goal of such experiments may be to observe the use and effectiveness of a proposed user interface by the test participants, rather than to measure the quality of an entire system.

The name of the experiment comes from L. Frank Baum's 1900 novel The Wonderful Wizard of Oz, in which an ordinary man hides behind a curtain and uses "amplifying" technology to pretend to be a powerful wizard.

John F. Kelley coined the phrases "Wizard of OZ" and "OZ Paradigm" for this purpose circa 1980 to describe the method he developed during his dissertation work at Johns Hopkins University. During the study, in addition to one-way mirrors and other techniques, there was a blackout curtain separating Kelley (the "Wizard") from the participant's view.^{[citation needed]}

The "Experimenter-in-the-Loop" technique had been pioneered at Chapatis' Communications Research Lab at Johns Hopkins as early as 1975, three years before Kelley's arrival. W. Randolph Ford used the experimenter-in-the-loop technique with his CHECKBOOK program, wherein he obtained language samples in a naturalistic setting. In Ford's method, a preliminary version of the natural language processing system would be placed in front of the user. When the user entered an unrecognized syntax, he would receive a "Could you rephrase that?" prompt from the software. After the session, the processing algorithms would be modified to address the newly obtained samples, and another session would take place. This approach led to the eventual development of his natural language processing technique, "Multi-Stage Pattern Reduction". Ford believed that Kelley coined the phrase "Wizard of Oz Paradigm" to describe a technique employed at least twice before Kelley began his work. Another team, Allen Munro and Don Norman from the University of California, San Diego, (Bobrow, et al.) used a similar technique to model a natural language understanding system at the Xerox Palo Alto Research Center circa 1975.

In that employment, the wizard sat at a terminal in an adjacent room separated by a one-way mirror so the subject could be observed. Every input from the user was processed correctly by a combination of software processing and real-time wizard intervention. As the process was repeated in subsequent sessions, more and more software components were added, and the wizard's role was gradually reduced. Eventually, the machine reached a point at which it could be left unattended, enabling the wizard to validate the final system's unattended performance.

In their 1985 University of Michigan technical report, Green and Wei-Haas state the following: The first appearance of the "Wizard of Oz" name in print was in Jeff Kelley's thesis (Kelley, 1983a, 1983b, 1984a). It is thought the name was coined in response to a question at a graduate seminar at Hopkins (Chapanis, 1984; Kelley, 1984b). "What happens if the subject sees the experimenter [behind the "curtain" in an adjacent room acting as the computer]?" Kelley answered: "Well, that's just like what happened to Dorothy in the Wizard of Oz." And so the name stuck.