Before revision: History 175px| Francis Galton's correlation diagram, 1875 Quantitative psychology has its roots in early experimental psychology when, in the nineteenth century, the scientific method was first systematically applied to psychological phenomena.
After revision: thumbnail|History 175px| Francis Galton's correlation diagram, 1875 Quantitative psychology has its roots in early experimental psychology when, in the nineteenth century, the scientific method was first systematically applied to psychological phenomena.