PERT and CPM are important tools for planning, scheduling and coordinating the overall activities of large-scale projects. PERT was developed by a group of engineers from the US Navy when they were in charge of Missile project in the year 1958. Although these two techniques (PERT and CPM) were developed independently, they are only superficially different. There are so many similarities between these two techniques and now-a-days these techniques together are called as critical path analysis (CPA). PERT and CPM techniques are usually discussed in project management training.
Following are the basic differences between PERT and CPM:

  • Program Evaluation Review Technique is event-oriented, while CPM is activity oriented.
  • Activity times in CPM technique is related to costs, but this is not the case with PERT as it is event oriented.

Now, as a project manager you need to know the basic concepts of PERT, and to estimate schedule and cost through PERT. There are three basic scenarios, based on the same you could derive the schedule and cost estimation:

Most Likely Time (m): This estimation can be done based on the strength of the resources available and it is the real picture and you can say it is the trend in the organization. You can guess this from the previous project files available in your organization from the previously completed projects. Most likely this indicates the time taken by an activity  to complete the task. It is very similar to heuristic estimation.
Optimistic Time (o): Estimation done on the best scenarios available in your project.  You as a project manager think that the activity could be finished a bit earlier if everything goes in order. This must be less than ‘Most Likely Time’.

Pessimistic Time (p): Estimation done on the worst scenario could be in your project. You as a project manager think that the activity could take more time than estimated due to several reasons. This must be more than ‘Most Likely Time’.

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There is simple formula based on the three above-mentioned scenarios available through which you can appropriately make estimation.
Estimated Activity Durations (EAD) =          
If you see this formula, you will be getting the estimated durations equal to most likely durations. This is a very useful formula that should be used by a project manager. In this way you can derive the durations of each and every activities of work breakdown structure.
You need to create a sequence among the activities with durations above it to get overall project durations. A network consists of series of activities called paths of network. Addition of durations of activities on a path gives the total durations of the path. The path with the longest duration determines the overall duration of the project. And this path is called the Critical Path. It is nothing but the longest path with shortest durations within which a project can be completed. It is very critical for a project manager to identify the critical path for his/her project.
If a project manager needs to fasten the delivery schedule, s/he needs to concentrate on critical path. S/he may think of schedule compression on the activities which is on critical path. So, you can understand how useful and valuable this path is for your project.
Remember, there is zero float on the activities which are identified in critical path. A Project manager needs to understand what is float and how it is useful for the project execution. A float is nothing but a leeway available to the project manager to have more flexibility in allocation of resources. There are generally two major types of float:
Total float: It is a time duration by which an activity can be delayed or extended without delaying or extending the overall project duration.
Free float: It is a time duration by which an activity can be delayed or extended without delaying the successor’s activity.
Determination and knowing the floats in your project is very much required for a project manager. Upon this information, s/he can plan the project more effectively, mainly when there is some additional request from the customer to complete the project a bit earlier.
In other words, float is the maximum leeway available to an activity when all preceding activities occur at the earliest possible time and all succeeding activities occur at the latest possible time. Thus, it could be obtained by subtracting the earliest possible time from its latest allowable time finish time.
So, you can say Float = Late Finish – Early Finish or you can say Float = Late start – Early start. You can derive the value of float in both ways.
So, the bottom line is, PERT and CPM are the two scheduling methods available for a project manager, which s/he need to derive correctly. Again, as project planning is iterative in nature, these values can be modified at the later stage of the project. But, your concept should be very clear on these methods and at least theoretical knowledge is expected from a project manager in this regard.

PMP is a registered trademark of the Project Management Institute, Inc.

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