Posted in Project Management, Software | Posted on 13-12-2009 | 11,148 views|
PERT is a technique to depict project activities and to calculate duration of each activity keeping uncertainty/risk in mind, PERT (Program Evaluation and Review Technique) was first develop in fifties (1950s), as a method to estimate activity duration, it is often referred to as PERT Chart, PERT uses indeterministic methodology to calculate duration, by trying to anticipate three estimates, worst case scenario represented by pessimistic value or longest duration an activity will take to finish, best case scenario represented by optimistic value or shortest duration an activity will take to finish, third factor is the most likely estimate, the 3 factors feed into a formula which takes probability into consideration to calculate the duration of the activity, the formula is:
For example if you are going to work on a powerpoint presentation, and you expect to finish it in a 2 days, that is not a good indicator because if anything can go wrong it will go, and often time you will not be able to finish it on the promised date, so you have to think for a while trying to come up with the shortest and longest duration in which you can finish the presentation, so if you feed into the formula these values (m = 2, p=5, o=1) this will give you 2.33 days.
A project manager should always use this technique in order to manage customers’ expectations, furthermore a resource has to be aware about it, because what basically happens in most of organizations is that most of professionals are just giving one estimate based on personal experience without considering potential risks, while the one can always buffer the estimate, yet it will never give a good indicator. in the following section I will show you how to use PERT technique using Microsoft Project
- Open Microsoft Project 2007, enter a list of tasks (always the very first task should be identifying tasks, ideally should be grabbed from a WBS created prior to creating the project schedule), in this case I will use a simple SDLC model, so you can enter Analysis, Design, Build, Test, and Deploy, add a top level task called SDLC
- Select the five tasks you just added and indent them to make the SDLC the summary task (you can use the Alt+Shift+Right hotkey right away).
- Do not enter any duration for the tasks, keep the default of Microsoft Project (1day?)
- Keep the five tasks selected and hit the link button on the toolbar or use the Ctrl+F2 hotkey
- Save the file and call it PERT.mpp
- Right click the toolbar area and select PERT Analysis, a new tool bar consisting of 7 buttons will show
- Select the last button called PERT Entry Sheet, the GANTT view will change to A PERT Entry Sheet including all the inputs required to the above formula, as shown in the below screen shot
- Go through each task and enter Optimistic, Expected, and Pessimistic Durations (do not enter any values in duration column)
- As soon as you are done with entering the values, click Calculate PERT button from the toolbar, you will get a warning box explaining that you must have entered the 3 estimates for each task, and that Microsoft Project will ignore any task which does not have all the 3 estimates entered, moreover it will inform you that the Duration1, Duration2, Duration3, Start1,Start2,Start3,Finish1,Finish2,Finish3 are used to store the 3 estimates you just entered and those fields will be overwritten as soon as the formula is applied, you just need to click Yes, but it is always good to read warning messages!
- Duration field is filled, and then you can know the best, worst, and most likely duration for your entire project!
If you switch now to the GANTT view you will see that the duration field has been populated with the PERT estimate and voilà
PERT FAQs (Frequently Asked Questions
- Do I have to use Microsoft Project to use PERT or can I use a spreadsheet ( Excel or Google Docs )?
You can use any software to calculate PERT, you can even use a calculator, the reason Microsoft Project is highly recommended is that it has a robust scheduling engine, and it will always give you the accurate PERT estimate taking into consideration relationships between tasks, for example if you have two parallel tasks you have to take care about that manually because the spreadsheet application will not be able to handle that, while Microsoft Project will efficiently handle this sort of situations
- If I use PERT technique, will this ensure that the project will be delivered on time?
PERT is just a formula that uses whatever estimates you give to calculate the PERT estimate, however if you are not doing some effort to estimate pessimistic, optimistic, and most likely before you use the formula, chances are the formula will not be helpful in this case, also using PERT is not enough, you have to add some contingency in your schedule in order to avoid any inaccuracy results in using PERT, if you use a contingency this will boost delivery on time probability
- What are the other buttons in the PERT Analysis toolbar?
The buttons offer different ways to customize the way PERT works, they are respectively:
- Optimistic Gantt: will show a table including optimistic duration, optimistic start, and optimistic finish and the Gantt chart will show based on optimistic values only
- Expected Gantt: will show a table including Expected duration, Expected start, and Expected finish and the Gantt chart will show based on Expected values only
- Pessimistic Gantt: will show a table including Pessimistic duration, Pessimistic start, and Pessimistic finish and the Gantt chart will show based on Pessimistic values only
- Calculate PERT: the button you used before to calculate PERT after entering all required estimates
- Set PERT Weights: Used to change the formula weights, you can change the weight value of each parameter
- PERT Entry Sheet: used to rapidly enter the values for each task before calculating PERT
- Should I change the weights of PERT formula?
The default formula assigns a probability percentage of 16.6% for the pessimistic and optimistic estimates, while it assigns 66.66% for the most likely estimate, however this highly depends on the project risk and also on the organization in which the project is getting implemented, hence there is no hard and fast rule to change the weights, it depends from one project to another, however most project managers use the formula as is
- What is the difference between PERT and CPM (Critical Path Method)?
While both techniques are very similar, PERT uses 3 point estimates and in fact sometimes it called 3 point estimate technique, while CPM uses one point estimate, so PERT cares more about risk while CPM does not
Next time you get asked about an estimate, think for a while and try to apply PERT technique, whether you are a project manager or not you will definitely need to use it to have better estimates.
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