PERT versus CPM
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Research as shown that the most common project estimation technique employed by project mangers include PERT and CPM. PERT is an acronym that stands for Program Evaluation and Review Technique. According to Scott (2006), this technique provides definitive estimates of how long it would take to complete tasks. It involves generation of three estimates to come up with a final estimate. The three estimates include; optimistic time, pessimistic time and most likely time. The most optimistic case (O), gives the best case estimate of time to complete a task and it indicates a situation when everything is working right.
The most pessimistic case (P), gives the worst case estimate of time to complete a particular task and it indicates a situation of the worst occurrence. The last estimate is the most likely case (M) which gives the normal estimated time to complete a task. Basically, this is the time expected to complete a task given the normal problems and opportunities. The three estimates are combined to create a single number that best describes how long it would take to complete a given task and then a weighted average of the three estimates is created with the most likely estimate being four-times than either optimistic or pessimistic estimate.
PERT estimation technique can be used in a number of situations. For instance, it can be used in estimating the feasibility of a given project in terms of cost and scheduled time for its completion. Kheter (2010) highlights the advantages and disadvantages of this method which can be used to determine the circumstances under which the method can be applied. One of the advantages of this method is that it has got added flexibility of fast tracking or slowing down the project as required. Therefore, where flexibility is of essence, PERT estimation method can be applicable. The other advantage is that it helps one to arrive at a realistic starting or ending date for project activities. This makes the method more appropriate in determining the most probable duration that a given project would take. This method also facilitates identification of a critical path thus reducing the overall project risk.
Therefore, in situations where a project manager wants to identify and minimize risks, this method can be appropriate. However, due to its disadvantages, as pointed out by Kheter (2010), it may not be suitable to use where the number of tasks or activities and dependencies are many. This is because as these activities and dependencies increase, complexities emerge thus increasing the risk of project failure. In addition, its lack of objectivity in the criteria used for defining initial optimistic and pessimistic estimates limits one to rely on it in future as the same criteria may not be applicable at that time. Finally, in situations where one may want the actual estimate of time needed to complete a given task, this method may not be reliable as it may lead to under-estimation of actual project time in some instances.
CPM is another commonly used estimation technique and it’s also an acronym that stands for Critical Path Method. It is a step-by-step technique used in project planning and defines critical and non-critical tasks with an aim of preventing time-frame problems and project failure (Linda and Brennan, 2006). According to the writers, this method is best suited to projects consisting of numerous activities that interact in a complex manner. Unlike PERT which uses three estimates, CPM uses a single point estimation of the completion time. In CPM, activities can be determined whereas in PERT, activity estimates are based on probability.
As earlier stated, this method is best suited to projects that have got numerous activities that interact in a complex manner. This adds to many other advantages of this method as highlighted by Shari and Rosalind (2005). The method shows the activities that are critical to maintain the schedule and which are not. Therefore, in situations where one wants to determine the most important activities to focus on, this offers the best option. Also, using this method, a project manager can determine the actual date for completing each activity and also compare what should be happening with what is taking place and take the corrective measures.
Therefore, in a situation where one would like to have good control and monitoring of the project, this method can be applicable. This method also provides a graphical view of the project in which dependencies are displayed to help in scheduling. This helps in the planning of the project thus making it reliable. In addition, activities and their outcome can be shown as a network. It evaluates the activities that can run parallel to each other. Therefore, this method can be applicable in situations where there are numerous and complex activities that are to be carried out together.
However, in situations where scheduling of personnel or the allocation of resources is needed, this method is not suitable as it does not support it. In addition, where larger projects are involved, it is not advisable to use this method as it can be complicated in such projects. Also, in cases where the project manger involved does not have sufficient knowledge of the method, it is not advisable to use the method as it is not always clear and needs to be calculated carefully.
Apart from PERT and CPM, there are other project management cost and scheduling techniques such as function point counting, weighted average (WAVE), bottom-up, top-down and extrapolation. However, though these techniques can also be applied to cost estimation and scheduling, they cannot override the benefits derived from using either PERT or CPM. As earlier indicated, they are the most common tools of estimation and are more reliable and accurate when it comes to estimation.
In conclusion, though PERT and CPM are the most common methods of estimation, the other methods cannot be underestimated as they can also offer some benefits which PERT or CPM cannot offer. However, I one would prefer PERT because it recognizes uncertainty in project time estimation and cost.
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