Critical Path Method as tool for project execution

Meaning:

Critical Path Method (CPM) is commonly used with all forms of projects, including construction, aerospace and defense, software development, research projects, product development, engineering, and plant maintenance, among others. Any project with interdependent activities can apply this method of mathematical analysis. The term is generally applied to any approach used to analyze a project network logic diagram.

Process:

The essential technique for using CPM is to construct a model of the project that includes the following:

1. A list of all activities required to complete the project
2. The time that each activity will take to completion, and
3. The dependence between the activities

Using these values, CPM calculates the longest path of planned activities to the end of the project, and the earliest and latest that each activity can start and finish without making the project longer. This process determines which activities is "critical" hand which have "total float" i.e., can be delayed without making the project longer.

Different paths for completion:

In a project execution, a critical path is the sequence of interlink  activities which add up to the longest overall time alloted.. This determines the shortest time possible to complete the project. Any delay of an activity on the critical path directly impacts the planned project completion date .A project can have several, parallel, near critical paths. An additional parallel path through the network with the total durations shorter than the critical path is called a sub-critical or non-critical path.

Prioritization of activities:

These results allow managers to prioritize activities for the effective management of project completion, and to shorten the planned critical path of a project by pruning critical path activities, by "fast tracking" (i.e., performing more activities in parallel), and/or by "crashing the critical path" (i.e., shortening the durations of critical path activities by adding resources). Originally, the critical path method considered only logical dependence between terminal elements. Since then, it has been expanded to allow for the inclusion of resources related to each activity, through processes called activity-based resource assignments and alloted resources.

Resource leveled schedule:

A resource-leveled schedule may include delays due to resource bottlenecks (i.e., unavailability of a resource at the required time), and may cause a previously shorter path to become the longest or most "resource critical" path. A related concept is called the critical link which attempts to protect activity and project durations from unforeseen delays due to resource constraints.

Monitoring of projects:

Since project schedules change on a regular basis, CPM allows continuous monitoring of the schedule, allows the project manager to track the critical activities, and alerts the project manager to the possibility that non-critical activities may be delayed beyond their total float, thus creating a new critical path and delaying project completion.

Currently, there are several software solutions available in industry that use the CPM method of scheduling.

Flexibility:

A schedule generated using critical path techniques often is not realized precisely, as estimations are used to calculate times: if one mistake is made, the results of the analysis may change. This could cause an upset in the implementation of a project if the estimates are blindly believed, and if changes are not addressed promptly. However, the structure of critical path analysis is such that the variance from the original schedule caused by any change can be measured, and its impact either ameliorated or adjusted for. Indeed, an important element of project postmortem analysis is the As Built Critical Path (ABCP), which analyzes the specific causes and impacts of changes between the planned schedule and eventual schedule as actually implemented.

These results allow managers to prioritize activities for the effective management of project completion, and to shorten the planned critical path of a project by pruning critical path activities, by "fast tracking and/or by "crashing the critical path" PERT. PERT facilitates identification of early start, late start, and slack for each activity.

Advantages:

1. PERT provides for potentially reduced project duration due to better understanding of dependencies leading to improved overlapping of activities and tasks where feasible.
2. The large amount of project data can be organized & presented in diagram for use in decision making.
3. There can be potentially hundreds or thousands of activities and individual dependency relationships
4. The network charts tend to be large and unwieldy requiring several pages to print and requiring special size paper
5. The lack of a timeframe on most PERT/CPM charts makes it harder to show status although colors can help.
6. When the PERT/CPM charts become unwieldy, they are no longer used to manage the project.

Differences between plan and execution:

During project execution, however, a real-life project will never execute exactly as it was planned due to uncertainty. It can be ambiguity resulting from subjective estimates that are prone to human errors or it can be variability arising from unexpected events or risks. And Project Evaluation and Review Technique (PERT) may provide inaccurate information about the project completion time for main reason uncertainty. This inaccuracy is large enough to render such estimates as not helpful.

Maximization of solution;

One possibility to maximize solution robustness is to include safety in the baseline schedule in order to absorb the anticipated disruptions. This is called proactive scheduling. A pure proactive scheduling is a utopia; incorporating safety in a baseline schedule that allows coping with every possible disruption would lead to a baseline schedule with a very large make-span.