Critical path method

The critical path method or critical path is an algorithm used to calculate times and deadlines in project planning. This calculation system known by its acronym in English CPM (Critical Path Method), was developed in 1957 in the United States of America, by an operations research center for the firms Dupont and Remington Rand, seeking the control and optimization of costs through proper planning and scheduling of project component activities. Another major project of that time, the "Polaris" originated in 1958 the creation of one of the critical path programming methods, known by the name of PERT (Program Evaluation and Review Technique).

In project administration and management, a critical path is the sequence of the terminal elements of the project network with the longest duration between them, determining the shortest time in which it is possible to complete the project. The duration of the critical path determines the duration of the entire project. Any delay in an item on the critical path affects the planned completion date of the project, and there is said to be no slack on the critical path.

A project can have multiple parallel critical paths. An additional parallel path through the network with the total duration close to that of the critical path, although necessarily less, is called a sub-critical path.

Originally, the critical path method considered only dependencies between terminal elements. A related concept is the critical chain, which aggregates resource dependencies. Each resource depends on the handler at the moment where the critical path occurs.

Unlike the Program Evaluation and Review Technique (PERT), the critical path method uses deterministic times, while PERT uses probabilistic times from three estimates. However, the development of a project based on CPM and PERT networks are similar and consist of:

• Identify all activities which involves the project, which means, to determine relations of precedence, technical times for each of the activities.
• Building a Network based on nodes and activities (or arches, according to the most used method), which involve the project.
• Analyze specific calculations, identifying the critical route and the slackness of the activities that make up the project.

In practical terms, the critical path is interpreted as the maximum dimension that the project can last and the differences with the other paths that are not the critical one, are called slack times.