In the life sciences, psychology, and large parts of the other social sciences, the ideal experiment is a comparative experiment with randomly composed experimental and control groups. Historians and practitioners of these sciences generally attribute the invention of this "random group design" to the statistician R. A. Fisher, who developed it in the 1930s for agricultural research. This essay argues that the random group design was advanced in psychology before Fisher introduced it in agriculture and that in this context it was the unplanned outcome of a lengthy historical process rather than the instantaneous creation of a single genius. The article analyzes how the random group design came about bit by bit when methodological practices from nineteenth-century psychophysical laboratories were gradually adapted, extended, and codified by twentieth-century educational psychologists to support procedural objectivity in educational administration. In passing, the article also amends the received historiography of the separate elements of randomization and control groups.