Both methods were originally presented in machines, and both were later presented in book form. Both systems were to an extent student centered. They were ways of teaching individual learners who worked at their own pace. Both systems (in different ways) used knowledge of results to promote learning. In both systems the content was pre-tested to identify problems and iron them out. Both systems emphasised clear learning objectives. Progress in learning was measured by pre- and post-tests of equivalent difficulty. Many practical tests showed the effectiveness of these methods.