The critical path technique had its origins from 1956 to 1958 in two parallel but different problems of planning and control in projects in the United States.
In one case, the US Navy was concerned with the control of contracts for its Polaris Missile programme.
The contracts comprised research and development work as well as the manufacture of component parts not previously made. Hence neither cost nor time could be accurately estimated, and completion times therefore had to be based upon probability.
Contractors were asked to estimate their operational time requirements on three bases: optimistic, pessimistic, and most likely dates.
These estimates were then mathematically assessed to determine the probable completion date for each contract, and this procedure was referred to as “Programme Evaluation and Review Technique”, abbreviated to PERT.
PERT reduced the development tie for the Polaris missile by over two years, a reduction of some 45%. Parallel Programme
The development of Critical Path Method (CPM) was enhanced by the Du-Pont company based in Newark, Delaware, who set up a group to study the possible application of new management techniques to the Company’s engineering functions.
One of the first areas considered was the planning and scheduling of construction projects. The group had a UNIVAC 1 computer at its disposal and decided to evaluate the potential of computers in scheduling construction work.
Mathematicians worked out a general approach, theorising that if the computer were fed information on the sequence of work and the length of each activity, it could generate a schedule of work.
In early 1957 with the help of Univac Applications Research Centre, the original conceptual work was revised, and the resulting routines become the basic CPM.
No fundamental changes have been made in this first work.
In December 1957, a test group was set up to apply the new technique, and then called the Kelly-Walker method. The assignment was to plan the construction of a $10 million chemical plant in Louisville, Kentucky.
The network diagram for the project was restricted to include only the construction steps. The project was analysed starting at the completion of its preliminary design. The entire project was subdivided into major areas of scope, and each of these areas was analysed and broken down into the individual work activities. These activities were diagrammed into a network of more than 800 activities. The method was hailed as a success and Du Pont invested heavily in CPM for another project in July 1958 valued at $20m.
Advantage of Critical Path Method
A fully developed critical path network is a logical mathematical model of the project, based upon the optimum time required for each work process and making the most economical use of available resources (labour, plant, materials). It has therefore been tuned to the individual problems of the particular project and may be detailed as desired to suit the anticipated conditions and hazards.
During execution of the project, it permits systematic reviewing of current situations as they arise, so that allowance can be made for the effects of uncertainties in the original planning, as well as enabling a revaluation of future uncertainties to be made, and remedial measures initiated for those operations – and only for those activities – that require correction or acceleration. It is indeed significant that where the critical path method has been introduced, considerable reduction of project times and costs have resulted. In the United States its use in the construction industry has led to decreases of up to 20% in project times over similar projects not employing CPM as a management tool.
The objectives of CPM can be described as follows:
“The fostering of increased orderliness and consistency in the planning and evaluating of all areas in the project”.
The basis for the CPM network approach remains essentially unchanged from its earliest formulation, and has survived every test, extension, improvement, change in format, and manipulation it has undergone.
Although network techniques are basic and logical, assimilation of the concept does take time and experience.
The principle danger in the continuing extension of and experimentation on network techniques is that the basic framework (the network) might be obscured or lost. Thus far, the semanticists have not been able to accomplish this, nor have they deliberately tried. On the contrary, the strength and effectiveness of the network approach have tended to delay the development of extensions. Extensions by the very nature of their complexity tend to run headlong into the law of diminishing returns.
What Network Analysis Does
Network Analysis techniques achieve their purpose in three broad steps.
(a) They present in diagrammatic form a picture of all the activities to be done and their dependency on one and another. The way in which this is done is to construct what is known as a “network diagram” in which each job is represented by a box. The way in which the boxes are linked indicates the dependencies of the jobs on each other.
(b) They consider the limitations imposed by the availability of labour, plant and materials and, in view of these, estimate the time required to do each job.
(c) They apply the estimated duration to the network diagram, and then analyse the network. Analysis in this case means the calculation of the total length of time in each path through the network.
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