Critical Path Method (CPM)
Critical Path Method or CPM, is a tool used in the planning and management of different types of projects such as construction, software development, research programs, product development, sales promotion and etc. It is essentially a mathematically based algorithm of network diagrams that depicts the sequence and interrelation of all the component parts of a project and is well suited for the construction industry. In comparison to the conventional methods of planning and scheduling of construction works - bar charts and progress diagrams - Critical Path Method proves to be more useful and precise as it allows the quick evaluation and comparison of alternative work programs, construction methods, types of equipment and etc.
The time-cost problem in the construction sector is forever debatable and has an infinite number of solutions. Today, the Critical Path Methods provides a systematic procedure of correlating cost and time of each activity involved in the construction project, to provide an optimum solution. It is a crucial tool used in construction management as it provides varying degree of involvement by the management to suit the needs and objectives of the project.
Historical Background
The critical path technique had its origin from 1956 to 1958 in two parallel but different problems of planning and control in projects in the United States.
In the first case, the US Navy was concerned with the control of contracts for its Polaris Missile program. As the contracts comprised of research, development and manufacture work of newly developed parts, the probability of completion dates for each contract was determined. This procedure was referred to as Program Evaluation and Review Technique, abbreviated to PERT. PERT did not originally include cost estimates, but subsequent inclusion of cost data in the system is called PERTCO, that is PERT with costs. It is therefore important to understand that PERT systems involve a probability approach and are better suited for projects where big uncertainties exist.
In the second case, the EI du Pont de Nemours Company was constructing huge chemical plants in America. These projects required that both time and cost be estimated accurately. This method of planning and control that was developed was initially called Project Planning and Scheduling (PPS), which covered the design, construction and maintenance of large and complex projects using realistic estimates of time and cost. This method has since been developed into Critical Path Method (CPM).
Critical Path Method Terminology
Planning
Planning is the process of choosing the one method and order of work to be adopted for a project from all the various ways and sequences in which the optimum result can be achieved. This is schematically shown on the CPM network diagram.
Scheduling
Scheduling is the determination of the timing of the operations comprising the project and their assembly for the overall completion time. This can only be done after the project plan has been defined and modelled as a network diagram.
Network Diagram, Activity and Event
A network is a flow diagram constituting of activities and events that are connected logically and sequentially.
Well defined jobs or tasks are called activities. These are represented by arrows in a network diagram.
The beginning or end of each activity constitutes an event of the project.In a network diagram, events are represented by nodes. These can be circular, square, rectangular or oval in shape.
Dummy
A dummy is a type of operation in the network which neither requires any time nor any resources, but is merely a device to identify a dependence among operations. A dummy is represented by a dashed arrow.
