The PERT Analysis – Part 1
September 1, 2014 Leave a comment
For every project, there are tasks that are outlined which serve as an indication of what needs to be completed and in what order to ensure the proper outcome and closure of the project. This can be expanded upon further in the concept of a program, which is a group of inter-related projects.
As a program and its constituent projects become more complex, inter-dependencies of tasks and project deliverables become an important piece of criteria to track. Successful completion of a project or program that has dependencies is a challenging endeavor. Care and diligence on the part of the project manager is paramount to ensure that specific dependencies are resourced and completed accordingly to ensure that other pieces of the overall project or program and also be initiated and eventually completed.
When it comes to properly cataloging and tracking project/program flow and interdependencies, it becomes imperative to do so effectively. With that being said, what is the best tool in the project manager’s arsenal to accomplish this task?
Program Evaluation and Review Technique
The Program Evaluation and Review Technique, or ‘PERT‘ for short, is a statistical tool and technique that is specifically designed to define, analyze, track and effectively represent the various constituent pieces of a project or program. It is mostly commonly used in conjunction with the critical path method (CPM) which is a technique used to schedule a set of project activities.
The main criteria for performing an effective PERT is to have the tasks, projects, interdependencies and timeframes all catalogued. Normally, this process will occur early in the project plan, regardless of inherent project process. The Work Breakdown Structure will generally drill down the project or program into its constituent task level activities. From there, each task can be given a timeframe for completion. (Note: for information on how to perform project scheduling and timeframe calculations, please consult the post: Project Scheduling-Tools & Techniques)
Once the various pieces are itemized, the PERT structure can be formed. The following are terminology definitions of the various pieces of the PERT that are used to produce the eventual visual chart are as follows:
PERT Event – Signifies the start or completion of an activity (task)
Predecessor Event – Precedes a given event but has no intervening events
Successor Event – Follows a given event but has no intervening events
A few key points in regards to the above definitions. The standard PERT Event consumes no resources and takes no time; it is merely used to signify the node of completion for various tasks. With regards to the predecessor and successor events, note that they can have any number of predecessors or successors within the entire structure.
Once the key attributes of the PERT analysis are itemized and catalogued, an actual dependency chart can be drafted to signify the overall flow of the project or program. As an example, consider the following:
A project has six activities, labelled A, B, C, D, E and F. Each activity has been given the following timelines for completion:
Each of the activities have varying dependencies that are displayed graphically as follows:
Once the activities are accurately drafted, with their inherent completion times and dependencies provided, the overall graph yields the estimated time for completion of the project. This estimate is referred to as the ‘critical path‘ and it denotes the longest path and timeframe based on the chart. In order to determine the critical path, all paths from the chart are evaluated with their completion times added together. So as per the above example, the paths and their mathematical sums are as follows:
A->D = (3+2)= 5wks
B->E = (4+2) = 6wks
A->C->F = (3+1+3) = 7wks
Thus, the longest timeframe or ‘critical path’ is A->C->F which is 7wks. Which denotes the overall time to complete the project.
The PERT method is an excellent mechanism to graphically display the primary project dependencies and overall flow. It is a high level overview that gives a good summation of project estimated completion time and overall variance. While PERT is valuable in demonstrating specific dependencies and overall project flow, it may become cumbersome if there are copious interdependencies in the overall project or program. In those cases, it may be advantageous to create multiple PERT charts denoting various sections of the project and then have those critical path estimates serve up to a global PERT that demonstrates the overall project or program flow and completion.
In the next post, we will delve into PERT and critical path further by discussing floats, fast tracking and earliest/latest start and finish times.