During the post-war period, each of the U.S. military services developed its own model for training development, and all of them were based on the systems approach, a soft science version of systems analysis, itself an offshoot of operations research. The systems approach was viewed in the military as a paradigm for combining the human element with the machine elements in man–machine systems, an antidote to purely mechanistic thinking. From the entry of the systems approach into the field of educational technology, it was recognized by its advocates as a loose set of guidelines that were applicable to the complex problems of human learning only by analogy and not the sort of completely deterministic and tightly controlled methodology described by some of its detractors.