This paper describes the role of formative evaluation in the development of a museum videodisk project entitled "Earth Over Time," which was sponsored by 15 museums of the Interactive Video Science Consortium. Targeted for 10- to 12-year-old children who have come to the museum with their parents, this earth science videodisk covers plate tectonics, volcanoes, earthquakes, sea floor exploration, and shoreline erosion. A description of the exhibit design problems is followed by discussions of the phases of production--high level design, detailed design, production, and implementation--and the escorted trial technique used for evaluation during the design phases. An example of the evaluation procedure is provided, noting the particular emphasis on four criteria: appeal, comprehensibility, accessibility, and responsiveness. Finally, the implementation formative evaluation process--which utilized videotapes of users, touchscreen records from the computer, and interviews with a sample of users--is described. (4 references) (GL)