In 1956 James E. Kelley Jr. (of Remington Rand) and Morgan R. Walker (of DuPont) began developing algorithms for project scheduling at DuPont, building on work that had been done at the company during the Manhattan Project. The program was first trialled on plant shutdowns in 1957, and they published the first paper on critical path scheduling in 1959.
Critical Path Analysis involves finding the "critical path" of a project, which is the path that is the longest, and therefore the least tolerant of slippage. This can be done algorithmically by analysing the complete set of activities required, their durations, their inter-dependencies and their end-points such as deliverables. Any other milestone or deliverable in addition to the end of the project can also be so analysed.
The development of Critical Path Analysis and the Critical Path Method (CPM), requiring algorithmic calculations which though relatively simple are often large in number, were inevitably closely associated with the development of computers, being one of the major applications of early computing technology. The original systems were complex mainframe-type computers that took operators many months to learn how to use. In the 1970s and 1980s computer technology progressed and CPM systems were transferred to "mini computers". These were still very expensive and only large organisations could make use of them. Finally, with the development of microcomputers during the 1980s, the ability to apply CPM inexpensively to large scale projects became a realistic proposition.