CPM and PERT in construction projects are the tools used for efficient management of activities. CPM is Critical Path Method and PERT is Program Evaluation and Review Technique.
Critical Path Method (CPM)
The Critical Path Method (CPM) was first used during the overhauling of a chemical plant in United States in the year 1950 by Morgan R. Walker of DuPont and James E. Kelley, Jr. of Remington Rand. By using this technique of activity management, they substantially reduced the time of overhauling of the chemical plant.
Program Evaluation and Review Technique (PERT)
Program Evaluation and Review Technique (PERT) was first used in 1957 in United States for the design and development of the Polaris missile. With the objective of meeting a specified completion date for the project two years in advance of the earliest possible date predicted by traditional planning methods the U.S. Naval dept. devised this new technique.
CPM and PERT in Construction Projects
The difference between the CPM and PERT is that the PERT is mainly used where the time required for completion of each of the activities involved cannot be accurately defined nor are the resources to be used for the activity readily available. The events could however be readily definable.
For example, for a construction project type, which have not been completed anywhere in the past, when it is possible to say that with reasonable accuracy that an activity "A" has to be completed before activity "B", but the time required to complete the activity "A" is not known or the completion time of activity "B" is uncertain, in that case PERT technique is used.
This technique is based on the probabilities of completing each activity in time. The time is not of much importance in this technique, and completion of each event is taken care of.
This technique is therefore most suited to projects like research and development, investigation, design etc. Also PERT because of the large number of calculations involved is essentially a computer-based system.
CPM technique is used in construction projects based on the knowledge and experience of the past projects for predicting accurately the time required for various activities during the execution of the project. Time required for each activity is known and defined for the project.
Hierarchy of the construction project events are well defined and time of completion of the same is also defined. The total time required for the given project can be estimated based on this technique.
CPM is an activity oriented system as the times required for construction activities are estimated more accurately. CPM is used where activities are definable and measurable and minimum overall cost is of the utmost importance.
Most large projects can be portrayed by a graph or network of jobs. Characteristics of construction projects to make it amenable for analysis by PERT or CPM are:
(a) The project must consist of a well defined collection of jobs or activities which when completed will mark the end of the project.
(b) The jobs must be such that they can be started or stopped independently of each other within a given sequence, (e.g. certain continuous flow processes such as oil refining, where jobs or operations must follow one after another with essentially no time separation, are not amenable for analysis by PERT or CPM).
(c) The jobs are ordered i.e. they must be performed in Technological sequence ( e.g. the foundation of a wall must be completed before the wall can be built).
PERT and CPM are tools used for managing the construction project activities and if followed thoroughly, the construction project can be completed within the time limit and within the cost. But use of these tools does not guaranty the desired outcome due to bad management problems, natural calamities, strikes by labors etc.
Howsoever good a tool may be, its success depends on how well the tool is used. The CPM Network affords management with information for taking decisions and focusing its attention to essential operations.