pert program evaluation and review technique

Schedule baseline and cost baseline are the two most important performance measurement baselines for any project. Cost baseline helps you assess the cost performance of the project, while the schedule baseline involves the duration.

Clients are very interested in the schedule because any delay can affect their business. So, project managers should give schedule development a top priority.

Developing a schedule is a big task and requires a thorough knowledge of schedule network diagramming techniques and experience. 

A few diagramming techniques are:

  • Critical Path Method
  • Critical Chain Method
  • Program Evaluation and Review Technique

I have discussed the critical path method and critical chain method in other blog posts. Here, we will discuss the Program Evaluation and Review Technique.

What is Program Evaluation and Review Technique (PERT)?

PERT stands for Program (or Project) Evaluation and Review Technique. The US Navy developed this technique in 1957. This helps you find a duration estimate of activities when there are uncertainties.

It is a statistical tool and mainly used in research projects where you cannot predict the duration of an activity. Therefore, you plan your work based on the milestones. It helps you develop the project schedule for large, complex, and one time projects when no records are available. Here, time is important.

PERT is a Three-Point Estimation technique. This technique uses three different estimates to arrive at a final estimate. It is a weighted average technique that reduces bias and uncertainty from the estimation and improves accuracy.

These three estimates are as follows:

  • Most Likely Estimate
  • Optimistic Estimate
  • Pessimistic Estimate

This technique is based on Beta Distribution.

Most Likely Estimate (Tm)

The chance of completing an activity is highest within this duration.

Optimistic Estimate (To)

This is a best-case scenario. Here the estimate is determined by considering all favorable conditions. This is the shortest duration to complete the task.

Pessimistic Estimate (Tp)

This is a worst-case scenario. Here the estimate is determined by considering all unfavorable conditions; the longest time the activity may take to complete.

The formula to calculate the PERT is as follows:

PERT Estimate = (To + 4Tm + Tp) / 6

And

Standard Deviation = (Tp – To) / 6

PERT vs CPM

At first glimpse, you may think PERT is like the critical path method; however, it is not. A few differences are:

  • CPM is activity-oriented, while PERT is event-oriented.
  • Duration estimates are definitive in CPM but not in PERT.
  • The CPM diagram shows the activity on the node, while in the PERT diagram the activity is shown on the arrow. Nodes represent the milestones. Therefore, many experts call it an Activity on Arrow (AOA) diagram.
  • The PERT diagram has only a “Finish to Start” relationship, while the CPM diagram can have any dependency: Start to Finish, Finish to Start, etc.
  • On a CPM diagram, rectangles represent the nodes, while circles represent nodes onPERT diagram.

The critical path method is a deterministic model because the activities have fixed estimates. Any delay in an activity’s duration estimate will affect your schedule. The solution to this drawback is built into the PERT, which helps you build a schedule when a definitive time estimate is not available.

Benefits of PERT

The following are a few benefits of PERT:

  • Planning is easier.
  • Schedule uncertainty is reduced.
  • It is useful when few or no records are available.
  • The completion date is more accurate.
  • It helps optimize resources.

Limitations of PERT

The following are a few limitations of PERT:

  • PERT requires a subjective analysis of activities and the accuracy depends on these estimates. If team members who are finding these estimates are less experienced or biased, it might affect the schedule.
  • Managing the critical path is difficult in PERT because it might change before the project ends.
  • This is an optimistic model that assumes resources will be available.
  • Updating, amending, and maintaining the PERT diagram can be time and cost consuming.

Now, let us look at an example on PERT and Standard Deviation.

Question

Your team members tell you that an activity you are working on is most likely to be completed in 20 days. However, the worst case, it might take 30 days, and if all conditions are favorable, it might be completed in 15 days.

Determine the PERT time estimate for this activity.

Solution

We need the Optimistic Time, Pessimistic Time, and Most Likely Time for the activity to determine the PERT estimate.

The question says it is most likely that the task can be completed in 20 days, hence: Most Likely Time = 20 days

It also says that in the worst case it may take 30 days, hence: Pessimistic Time = 30 days

Finally, it says that if all conditions are favorable, it will take 15 days to complete the task, hence: Optimistic Time = 15 days.

Now,

PERT Estimate = [Optimistic Time + 4 X (Most Likely Time) + Pessimistic Time] / 6

= [15 + 4X20 + 30] / 6 = [15 + 80 + 30] / 6 = 125 / 6 = 20.83 days

Hence, the PERT estimate for this activity is 20.83 days.

Summary

PERT is a network diagramming technique that is used for large projects where uncertainties are high and time is the main constraint. This tool helps project managers to develop a sound schedule when there is little information on activity duration. In this diagram, nodes are milestones instead of an activity. The success of PERT depends on the experience of the professionals who have developed it. It is the job of the project manager to remove the biases from the estimation as this can affect your PERT schedule. 

How do you develop your schedule based on the PERT diagram? Please share your thoughts in the comments section.

Further reading: What is PERT?

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Speak Your Mind

    • If you must change to non decimal, as per my understanding you should go for 5. If an activity is completing in 4.5 days, you can not say it as 4.0 because you may be in trouble while competing the activity.

      However, 5 is safe.

  • Bro
    I want to know about this PERT Estimate = (To + 4Tm + Tp) / 6 formula
    Why we take “4tm” in the formula.
    Can you evaluate bro
    Please

    • This is weighted average, that is why we have given more weightage to optimistic time. Derivation of this formula is out of scope of the PMP exam.

  • Yes, you can but it depends on the project and deliverable. Many times milestones cannot be a deliverable but a point, specially in a research project.

  • Hi Fahad,
    Milestone vs Deliverable:
    A Milestone includes 1 or more Deliverable, a Milestone cannot be delivered in itself to the Client, as it is only a point in time on the schedule with zero duration, while the Deliverable is a product that will require delivery to the Client.
    Therefore, I would suggest using Deliverable rather than Milestone on the Node for PERT.
    Example: prelim design completion is a Milestone while prelim design report, plan etc are deliverables.
    Your Thoughts

  • Hi Fahad,

    We have a question where we need to calculate the most optimistic time. The most likely time is given as 20, most pessimistic is 60, and expected is 10. This yields a negative answer for the most optimistic duration (-80). Is that possible and what does that mean?

  • Hello Fahad,

    Just wanted to thank you for your clear and easy to understand comments helping all the interested parties, through your blogs.

    Keep on the good job, its much appreciated!
    AA

  • Hi

    I am giving exam this month end
    what different types of question will come on PERT ,CCM, and CPM or infact on Time management chapter .

  • I head Activity on Node method is not allowed in PERT, Only activity on arrow works
    IS it true, if yes why and if no then also y?

  • Hi there, so I have a project about PERT. The question is whats the most likely activity time for x activity (nearest day) be adjusted to if the probability of any path through the network being completed in 69 days must not be less than 0.94?

  • 1. Can I use Bottom-up Estimate for calculating duration of an activity?
    2. In your above notes standard deviation that is 2.5 where we use it, if iam not wrong in finding Early start that could be 18.33 and late finish could be 23.33 is that ok, or it has some other means, please let me know.

    • For the project duration, you can go for the bottom up estimation, not for an activity.

      It should be activity start date between 18.33 to 23.33

      • Fahed
        As per PMP book , we can use bottom up estimation in the calculation of activity resources or activity cost , how we can use it for the project duration ?

  • Hi Fahad,

    In PERT or AOA,which relationship is used.. Is it FInish to start or Start to finish.and please explain why..

      • As per your notes “PERT Diagram can only have a “Start to Finish” type of relationship, while the CPM method can have any type of dependency; e.g. Start to Finish, Finish to Start, etc.

        You replied to SWAPNA ” It is Finish to Start because PERT is an event oriented network diagram

        Which one is correct your notes or your reply to SWAPNA, please explain. Thanks

  • I did appeared in the PMP exam in July 2013 but unfortunately could not passed, can I take a date for my exam in 2015 year, am I still eligible to write an exam. I am already been doing preparation for PMP exam with the help of your exam preparing material.

    Waiting for reply

    Thank you very much
    Tauseef Qureshey

    • Since you attempted the exam on July 2013, it is now more than one year; therefore, you eligibility is expired.

      So, if you want to sit for the exam now, you have to re-apply for it.

    • In triangular distribution you use three parameters: minimum, mode (most likely), and maximum. PERT is based on the triangular distribution.

      In beta distribution you get a curved shape. You can say that the triangular distribution is a simpler form of beta distribution.

      • Traditional PERT is based on Beta Distribution. However, triangular distribution are also used. If not informed specifically in the question, I’ll say use beta distribution formula, not triangular distribution.

  • Hi
    I am currently tyring to get ready for the PMP exam and following you regularly. You are helping out us a lot throughout this journey 🙂 just I would like to ask you that how standart variation can be used in a project? Does it mean that in your example, this job can be finished between 18.33 days and 23.33 days? And can I use this numbers as early start and Late finish in AON?

  • SA Fahad,

    I took my exam yesterday, and did really well. I prepared for the exam well by not only reading Kim Fieldman’s and other book, but also reading and clarify many topics from Pmstudycircle.com. Your journals were brief, clear, and with good examples. In my opinion, you should write a book …

    Thank you,
    Hotaki

    • WAS Hotaki,

      Thanks for your comments and I am happy that my blog helped you pass the exam.

      I have already written an ebook on pmp practice questions, and second ebook is on its way…

      🙂

      Fahad

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