PERT: Estimating Complex tasks & Uncertainties

PERT
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Introduction

We all know there are two critical questions that are associated with every project that we get:

  • How much is the project cost?
  • When will the project get finished?
The response to the above question will be quick if we are very certain about the work items that need to be completed.

But what if there are uncertainties in the scope of the project?

Estimating a Job or Task becomes a nightmare if there are uncertainties in the deliverables work items. And Project Sponsors always required the estimate to be accurate, which is sometimes difficult with uncertainties.

Estimated time is important even if there are uncertainties in your project. Let’s take a look at a real-life situation.

We all have Google maps used for navigating or driving to a location, if you see Google shows you an estimated time based on the uncertainties like road conditions, traffics, etc. No matter what’s the uncertainty we always get a single number from Google as an estimate which most of the time is accurate.

How to estimate these uncertainties? PERT is the answer.

PERT is used both for estimating and also to define the schedule of the project. In this article, we will be discussing only how to use PERT in estimating tasks.

What is PERT?

Back in 1950, US Navy faced the issues while developing the nation’s first, submarine-launched, ballistic missile – Polaris. These issues were probable and improbable project outcomes for task outcomes.

The program needed a new method to create a schedule for the Polaris, which would create a risk-adjusted schedule that considers uncertain project outcomes for the given task duration.

The  Project Evaluation and Review Technique is founded to overcome these issues.

The PERT method is still widely used for project estimation by Project Managers/BA.

Lets understand

PERT wants you to come up with three estimates, considering the uncertainties and risks associated with the task.

These estimates are:

  • Optimistic (O) – The estimate assuming that all the resources are available and understood, for the task to be able to deliver.
  • Pessimistic (P) – The estimate assuming the worst-case scenario where all the uncertainty is considered, for eg: resource unavailability.
  • Most Likely (M) – This will be estimated based on the experience or on the fact that if there is any Risk encountered, the Project team can manage it as they would have handled a similar risk in the past.

After gathering the estimates, the expected completion time for a task can be calculated as below:

Understanding with Example

Suppose, you are going to get your Mobile phone fixed which has its screen broken and you tell your wife that you’ll be back in an hour. This announcement is purely based on assumptions, not on experience. Now Let’s put PERT into action and see how much you are confident about that promise you made to your wife.

Optimistic:

  • Reach the service center, following Google Map, and register the token for service desk: 10 min.
  • Wait for your Token no. to be called: 30 minutes. (Yes, truly optimistic)
  • Show the broken screen to the service engineer, understand the process and leave the phone for repair: 10 minutes.
  • That’s 50 minutes, which is well within the hour. (Leaving room to visit a flower shop to buy some flowers for your wife, necessary sometimes).

Total estimate: 60 minutes

Most Likely:

  • Reach the service center by following the Google map, there was a Roadblock in between so google has shown a long route: 20 min.
  • Register for the token: 5 min. (Kiosk system  was slow responding slow due to erratic network)
  • Wait for your Token no. to be called: 40 minutes. (Less optimistic)
  • Show the broken screen to the service engineer, understand the process and leave the phone for repair: 15 minutes. (Assuming engineer is taking time to check the phone and other issues in the phone if any)

Total estimate: 80 minutes

Pessimistic:

  • Reach the service center by following the Google map: 30 minutes. (Traffic situations in consideration, considering if there is a flat tire situation or any other non-probable events)
  • Register for the token: 15 min (Long waiting line and Kiosk is responding slow as well for each token to be raised).
  • Wait for your Token no. to be called: 60 mins. (Those crowd on the front…)
  • Show the broken screen to the service engineer, understand the process and leave the phone for repair: 15 mins

Total estimate: 120 minutes

Let’s see what PERT says, by when you will be back,
That means it would take approximately 80 mins for you to reach home. Now the question is How much confidence are you that you will reach in 80 mins (Approx.). For this, we will be using Standard deviation.
 

Standard Deviation (SD)

The standard deviation formula is,

Calculating Standard Deviation for the above problem,

That means it would take 80±10 mins you to reach home.

 

Conclusion

We all know estimation sometime gives us nightmare, and if there are uncertainties, that worsen the situation. I recommend the above estimate technique should be known to every individual. I have been using PERT since the beginning of PM Life.

The most important thing about PERT is, it’s easy to learn and will help you to make good and trustworthy estimates.

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Amit Nair

Amit Nair

Amit Nair is working as a Project Manager at Knoldus Inc. He has over 13 yrs of experience in managing Web Development and Mobile App Development projects. He has over 100 Projects in his portfolio. He likes to read a lot and recently started writing about his experiences in the Project Management domain.

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Written by 

Amit Nair is working as a Project Manager at Knoldus Inc. He has over 13 yrs of experience in managing Web Development and Mobile App Development projects. He has over 100 Projects in his portfolio. He likes to read a lot and recently started writing about his experiences in the Project Management domain.