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Pert Diagram and CPMPert History
The PERT chart is one such tool. Developed by the U.S. Navy in the late 1950s, PERT is an acronym for Program Evaluation and Review Technique, sometimes mistakenly called Project Evaluation Review Technique. There were thousands of contractors and agencies who the project managers had to handle for the development of the Polaris Missile. Projects of such mammoth sizes had no precedent in the history of man. There was no prior project management experience available to the managers to handle them. Ingenuity, coupled with determination to succeed against such odds gave rise to a clutch of management tools - all using the humble paper and pencil. At this time, the PERT chart was created. ResourcesCharting BasicsA PERT Chart is a visual tool that project management uses to schedule, organize, and coordinate activities of their project. In every project, a list of tasks is broken down (cf: Work Breakdown Structure), to complete in a definite sequence, which define the project. Resources required to complete each task - in terms of money, material, and manpower - are listed. Tasks are then sequenced one after another along a timeline. Tasks which can be done simultaneously, without having to depend on one another for completion, are noted. Milestones are penned down, which have to be reached while within the project. Creating a PERT chart then involves thinking through the project flow. The project manager creates circles or rectangles to represent events - or milestones - in the project. These are called Nodes of the PERT chart. Activities which must occur in order to reach a milestone are represented by arrows, and these are known as vectors in PERT terminology. Nodes are linked by the Vectors, and the direction of the Vectors indicates the sequence of activities. Together, the Nodes and the Vectors, along with the information that they carry, form the Chart or the Network Diagram. The first activity, or set of activities originates the Initiation Node. When multiple activities begin in parallel, the vectors are shown as emanating from the Node, outward. On each Vector is noted the following information:
Every activity ultimately results in some event or milestone. Correspondingly, every Vector ultimately terminates in some Node. The progression of cause-and-effect; that is, of activities as Vectors ending in events as Nodes continues; until we have a complete chart, wherein all the final activities converge at the completion Node. There would be some Vectors that do not require any resources. They start from one Node and terminate into another, in order not to break the flow of the network, and to ensure that the entire diagram has just one Node - the completion Node - which has no Node leaving it. These are called Dummy Vectors. (Otherwise there would be "hanging" nodes - events that seemingly have got nothing to do with the project completion!) Thus, a PERT chart affords the project management a bird's-eye-view of the entire project. Along with an associated Gantt chart, they are able to gauge where they stand in terms of the project being completed within the estimated time. Hidden DetailsIt is not unusual conduct periodic reviews during a project, or after every predefined set of milestones are reached. These review points may be given their place of honor in the PERT chart, by representing them with a Node, and having all preceding milestone nodes connect to them with the help of dummy Vectors. Progress Reviews may take anywhere between 1 day to 2 weeks or even longer. A prudent project manager has to build in this time consciously in their plan. Also, frequently, mundane tasks such as documentation, project report writing, and report reproduction are taken for granted. Managers diligent about the slightest schedule slippage must give these activities their deserved place in the PERT chart. This also helps in underlining the importance of such tasks to people who are supposed to carry them out. Critical Path Management (CPM)One of the best advantages of creating a PERT chart during project planning is the insight it provides on the Critical Path. In our network diagram above, a critical path is the longest path traversed from the starting Node to the ending Node, in terms of the total of the calendar time taken to achieve each intermediate Node. All activities on this Critical Path must be completed on schedule for the project to finish on time. Seen in this light, each activity in the Critical Path is a critical activity, to be given the utmost attention. Resources from tasks which are not critical can be reallocated to assist Critical Path elements should issues arise, or inevitably unforeseen conditions occur. The process of managing the critical path is is referred to as Critical Path Management, or CPM. On the other hand, an activity outside of the critical path "can" be delayed without the project getting delayed - for a certain amount of time. PERT experts call this the Slack Time for that particular activity. Sometimes, the Critical Path is not immediately obvious when viewing the Chart cursorily. In such situations, we can determine the following four parameters associated with each activity:
Slack for each activity is then computed by subtracting TEF from TLF. The slack thus calculated may be noted on each Vector in the network diagram. The critical path shall then be a path that traverses the diagram across those Vectors where slack is Zero. Project ProgressAfter the project plan is created, a thorough PERT chart is in place, customer approval obtained, and all resources have been allocated and scheduled, the project commences. As the project progresses, actual results start trickling in. These results could be activity progression on schedule, behind schedule, or ahead of schedule. When there is deviation from the estimated times, root cause analysis is performed, and appropriate corrections need to be carried out in the PERT chart. Similarly, as the project unfolds with the progression of time, activities that were originally thought of as outside the Critical Path might suddenly threaten to upset the completion date of the total project. On the other hand, time estimates that were presumed to be quite intelligent guesstimates - based on expertise, experience, and what have you - seem to appear hollow. An ever-alert manager keeps themselves on their toes, learning every day of the project, and applying this knowledge about the men, material and the ground-realities involved to further fine-tune the future activities down the network diagram. ConclusionOver time, the original PERT chart notation has been modified so that activities are denoted as Nodes rather than arrows. The manager may decide to use any of these alternatives, which best suit their particular situation. It is very easy for a project manager to get stuck in one single task or activity, or get bogged down with some particular event. There is always the risk of losing sight of the overall picture. To prevent this, one must continually consider the Critical Path and PERT chart. |
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