Construction Management >> Critical Path Method (CPM)

Critical Path Method (CPM) is an effective tool for scheduling the activities of a complex Project. It was developed by the Dupont corporation in 1957.

In CPM, the project is modeled as a network showing the relationship between different activities and events of the project. The network diagram is drawn with activity-on-arrow (A-O-A). In A-O-A network the events are represented by nodes, whereas the arrows represent the activities. A work breakdown structure (WBS) is essential to develop before the network, where  the whole project is divided into smaller sub-projects which are further divided into smaller activities. The duration of activities and the precedence (dependence of one activity over the other) relation between the activities is required to be known before developing the network .

CPM helps to find out the duration of the project. It tells about critical activities which should be completed on time otherwise the project will be delayed.

It is important to know following four parameters for each activity;

1. EST = earliest start time

2. EFT = earliest finish time

3. LFT = latest finish time

4. LST = latest start time

The critical path is the longest-duration path in the network. The total float or slack of all the activities along  the critical path must be zero. Total float is the difference between latest and earliest start times of the activity (it is also equal to the difference of latest and earliest finish times of the activity).

CPM is a deterministic model which does not take into account the uncertainties in the project completion time. A tool which considers the uncertainties in the project is known as PERT (Program Evaluation and review Technique).

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