Program evaluation and review technique (PERT)
Developed by Thomas Kalmus
The Program Evaluation and Review Technique (PERT) is a method used in project management for planning, scheduling and coordinating the tasks of a project. It is based on a network diagram analysis of the different tasks that are involved. PERT is specifically suited for large, complex and nonrepititive projects. The aim of this article is to give an overview of this method with examples of applications and a comparison to other existing network based project management tools. It will also explore the usage of this method in todays projects, its advantages and limitations.
Program Evaluation and Review Technique, as a network analysis tool, gives the possibility to analyze large projects task by task and to estimate the amount of time that will be spent on each task. It was first introduced in the 1950s by the US Navy, during the works on the Polaris missiles project. It was a large and complex project involving multiple contractors and an effective planning tool was a necessity to finish the project successfully. The project has been finished long before the deadline and most of the people attributed this success to PERT.
PERT has several concepts that need to be defined, to understand the whole method. Those are :
- Network diagrams
- Critical path
The activities are the tasks that need to be completed. Each task has a predefined duration, a priority, a cost and an amount of workers associated with it. On a network level, they are represented as nodes connected by arrows. They can have predecessors - other tasks that need to be completed before them - and successors - tasks that can start only once this one has finished.
The events are points in time marking a phase of an activity like the start or the completion. Events can relate to one or more activities.
A network diagram is a visual representation of a project's schedule. It presents all the information about the project on a time basis like the tasks, their starting/ending times, total duration. Using this kind of representation makes the whole management process much easier and less error-prone.
The critical path is the sequence of activities which add up to the longest overall duration. Hence it represents also the total time that will be needed to complete the project. Any delay that occurs in one of the activities on this path will affect the completion date.
The process of making a PERT analysis can be divided into 5 steps:
- Preparation of a list of all the activities that will be involved in a given project
- Creation of a network diagram
- Estimation of the time required to finish each activity
- Calculation of the critical path
- Network analysis
PERT starts with a standard step of defining the activities that will be involved in the project. This information is presented in a box representation showed in figure 1. This box will later be filled with other time informations about the project. For instance the earliest start time, which is self-explanatory, is the time when the earliest date for an activity to be started. This goes in pair with the earliest finish - earliest date when the activity can be finished. These two timings report the most optimistic scenario and aren't usually met. The same logic applies to the latest times, but those are considered as the pessimistic time. More about these times will be later. Float is basically the difference between the earliest times and the latest. Its purpose is too give an overview of the timing margin that a certain activity has.
Once the basic information about the activities has been gathered, the activities need to be put in order. The priority of the activity and the dependencies between them are taken into consideration. In the network, activities are represented as arrows, whereas the events are the nodes just like on figure 2. For instance, the preparation of the base of a building would be decoupled into "starting of preparation" and "end of preparations".
Estimation of the time
In this step, the amount of time that can be taken by each activity is estimated. This time can be determined using a probabilistic or a deterministic approach. Three times need to be defined :
- the optimistic time (short time)
- the pessimistic time (shortest time)
- the normal time (most likely time)
The optimistic time (O) is the shortest time an activity will take if everything goes right. In general, it is highly improbable that the project will finish in this time, due to all the other factors.
The pessimistic time (P) is the longest time that an activity will take. During the calculations, several scenarios need to be considered with their respective probability in order to have a quite precise value.
The normal time (M) is the most likely time of an activity. Usually, it is determined using the time other similar activities took in other projects. For instance, the time needed to build a cockpit for one aircraft might be based on the average time it took to build cockpits for comparable aircraft in the past. Using these three times one can calculate the expected time of an activity with the formula below : Expected time = O + 4M + P / 6
It is basically an average of all of the times mentioned, but the normal time is given a higher weight, since it is the most probable. The expected time will be later used in the PERT diagram.
Critical path estimation
Once all the time information has been gathered about the different activities, it is possible to estimate the total duration of the project and the starting and ending dates. Determining the critical path comes down to identifying the path that takes the most time. A project cannot be completed as long as its longest component isn't finished. Here are the steps to find this path :
- Step 1 : Starting from the first activity, write the starting and ending times, by adding the duration to the ending time of the preceding activity.
- Step 2 : Make the same thing but from the end and substract the times instead of adding.
- Step 3 : Having the starting times and ending times calculated in two different ways, we can identify the activities which have a null difference between the respective times. Those activities constitute the critical path.
Time cost trade-offs and resource allocation
Having a first estimate of the time necessary to complete the project, it is possible to make a few changes to the plan and compress it a bit. In this step, the manager can decide which activities could be finished faster by reallocating resources properly or increasing them.
Having a clear plan of action, the project can finally be started. Every activity is monitored and checked on the schedule. Thanks to a clear representation of the project, in any case, the schedule can be reanalyzed and resources can be managed properly, but this step might be quite time-consuming and can lead to errors. Sometimes, especially at the beginning of the project, one change in an activity can cause the modification of the whole network, leading to a recalculation of the whole plan, which might be longer than necessary.
Applications and limitations
PERT, like other methods, has its pros and cons. Its application can some projects bring a huge benefit, but can also make a disaster in others. The choice of this tool must be well thought before.
The advantages of PERT are :
- Compels to plan and predict project before start. The planning of the project is made in details, resulting in less surprises later.
- Better management of resources. Since we have a detailed plan, we can precisely manage all of the resources, sometimes leading to a reduction of costs of the whole project.
- Focus on the critical activities.
- Good control over the project from the beginning; weak points of the project located right from the start. Gives the possibility to adapt quickly to the situation
- Provides updated information about the project
- Clear view of the dependencies between activities
- Accurate time estimation thanks to the 3-way computation of the total timing.
PERT limitations are :
- Emphasis only in time and not cost. This method does not present a model to estimate the costs of the project, therefore it is totally dependent on the managers skills and assumptions.
- Lots of analysis during control. Although a good plan can reduce the time of the whole project, making such a detailed plan is time-consuming, which leads to no time gain at all.
- Errors in time estimation might lead to miscalculations. Even if the 3-way estimate was considered an advantages, it is still based on assumptions and data given by the manager.
- No representation of repetitive activities. Because the project is presented in a network form, a repetitive activity would be represented as an inifinite loop, which could prevent us to determine the critical path.
PERT is a tool that can be useful in big and complex projects like in the construction industry. As mentioned in the introduction, it was created during the Polaris missile project and had a huge popularity afterwards. But soon, project managers and companies have found that it is too ineffective and error-prone to use it, causing lots of failures. The main reason behind it was the lack of IT tools, which had not the same abilities as now. Another major point was the update step in this method, which requires to reorganize the whole plan and in the case of large projects, this can be a time consuming and confusing step.
Nowadays the software and hardware part is not a problem anymore. Many management software include PERT charts as part of their functionalities like Microsoft Project (see figure 3) or Oracle Primavera. PERT charts are used to visualize the dependencies between different activities. To see the timing and progress of the project another similar tool is widely used, the Gantt charts . More popular than PERT are the Critical Path diagrams, in which nodes represent the activities, a more readable representation.
Comparison to other tools
As mentioned, PERT belongs to the family of tools based on network analysis. This category includes also other tools like the Critical Path Analysis , the matrix-based Design Structure Matrix, petri nets. How do they differ from eachother?
PERT and Critical Path Analysis are highly related, since one of the steps in PERT is to find the critical path of the project. They differ mainly on the way the timing is estimated and the visualization. In the Critical Path Analysis the activities are represented as nodes, whereas PERT uses nodes to represent events.
Gantt charts are used to display the timing of activities and progress of the project (see figure 4). Timings in Gantt charts can be estimated in the same way as in PERT and the whole process of building Gantt charts is almost identical to PERTs process. The use case is the only difference; because Gantt charts have a simplier structure and a better visualization, their usage is much wider and they appear more often in software tools.
Design Structure Matrix (DSM) is a modeling technique based on network analysis that can be used for the design, development and management of complex systems. The essential difference between a PERT chart and DSM is the visualization. DSM matrices represent basically the same information as a PERT network, which is different activities and their connections. The advantage of this representation is its conciseness compared to PERT, where lots of space is required to have a readable network (see figure 5). However it doesn't give any information concerning timings of activities. It is less focused on the planning but more on the management.
Other modified versions
Although PERT is not as popular as it used to be, scientists have tried to enhance it in different ways, mainly the computation of the duration of activities, since its the weakest point of this method. Here is a short list of some other PERT versions :
- GERT - enhances PERT with the possibility of making loops and multiple branches, leading to different results.
- PERT II - usage of a stochastic approach to determine the time of an activity; usage of a cumulative distribution function to determine it.
- PERTBN (Bayesian network) - usage of bayesian networks to determine the duration of the activities.
- RPERT (repetetive PERT) - usage of PERT in repetitive projects.
- Responsibility Assignment Matrix (RACI Matrix)
- Scheduling techniques in Project Management
- Critical chain project management (CCPM)
- Critical Path Method in Construction Industry
- Critical Chain Project Management
- Metra Potential Method
- The Critical Path Method (CPM)
- ESSENTIALS OF MANAGEMENT, Andrew DuBrin, Section "Program Evaluation and Review Technique", General overview and presentation of the method with all of the steps.
- Programming and Scheduling Techniques, Thomas Uher,Adam S. Zantis, Chapter 13, presentation of the method with an emphasis on the event side compared to the critical path method.
- The Handbook of Project Management: A Practical Guide to Effective Policies, Trevor L. Young, presentation of the method from a visual side (box drawing and step by step filling)
- Entrepreneurship Development, S. Anil Kuma, presentation of the method with a discussion of the advantages and disadvantages.
- https://www.mindtools.com/critpath.html, comparison of the Critical Path Analysis and PERT, how they differ and how are they connected.
- Project Management Demystified: Today's Tools and Techniques, Geoff Reis, provides real life anecodtes about the method, discusses how it became famous and why it is less used now.
- https://www.mindtools.com/critpath.html, comparison of the PERT method and CPA
- Program evaluation and review techniques II (PERT II), Yakhchali, Siamak Haji, 2011, Discussion about the PERT II methodology, which introduces a stochastic approach to determine the time of an activity
- The project schedule management model based on the program evaluation and review technique and Bayesian networks, Zhao, Zhigang; Jin, Yong, 2011. Presents the usage of Bayesian networks to determine the probability of the time of a certain activity.
- RPERT: Repetitive-projects evaluation and review technique, Aziz, Remon Fayek, 2014. Presents a modified version of PERT, and a sample software implementation that enables the usage of this method. The software takes care of the calculations of the times.