# Describe the PERT technique of scheduling. Why would one consider using PERT instead of CPM?

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Can someone please explain to me the PERT technique for scheduling projects? Also, please explain why would a project manager consider PERT for scheduling his project instead of the critical path method (CPM).
asked 8 years ago edited 8 years ago

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Both PERT analysis and CPM are used during Time Management to help us define and develop the project schedule.

In order to do so, we should follow some steps (processes):
1. Define all the activities and milestones that are required to produce the deliverables of the project.
2. Take the activities and milestones and put them in the order they should be performed (sequence them). That is to make a network diagram which shows all the activities with their logical relationships (dependencies).
3. Then we estimate the resources needed to perform the activities, and finally...
4. We estimate the duration of all activities on the network diagram.

PERT analysis is a tool we use to calculate the duration (or cost) of project activities, while the CPM (Critical Path Method) is a technique we use AFTER we have found the duration of activities and involves determining the longest path through the network diagram (the critical path), the earliest and latest an activity can start, and the earliest and latest it can be completed.

Here's how we can find the critical path in the project schedule:

- We find out all the possible paths in the network
- We sum the duration of all the activities in each path
- The path with the highest sum is the critical path, which is the shortest time to get the project done.

PERT ANALYSIS:

With PERT analysis or three-point technique, estimators give a Pessimistic (P), a More likely (M), and an Optimistic (O) estimate (time or cost) for each activity. This ultimately provides a risk-based expected activity duration by taking either the average or the weighted average of the three estimates.

Activity Duration Estimate:

1. The first formula to calculate is the Expected Activity Duration (EAD)

2. The second formula we need is the one which calculates a range for an individual activity estimate. To do this, we need to know both the expected activity duration (EAD) and the activity Standard Deviation (SD)
SD=(P-O)/6
We calculate the range using EAD +- SD. The start of the range is EAD-SD, and the end of the range is EAD+SD.

3. The third formula we need is the Activity Variance (AV) which is the weighted average of the expected activity duration.
AV=((P-O)/6)^2 (That's the "SD" squared)

Note that all the above formulas relate to activities, rather than the overall project. But as a project manager, you need to understand how these ranges affect the estimate of the overall project duration. You can then use this knowledge to effectively address variations on your project.

Project Duration Estimate:

1. In order to find the overall project duration estimate, you should first find the Expected Project Duration (EPD); this is the sum of the Expected Activity Durations (EADs) for ALL activities on the Critical Path:

2. Then you find the Project Standard Deviation (PSD) for the project:
- calculate the Activity Variances (AV) for each critical path activity,
- add those variances - their sum equals the Project Variance (PV), and then
- take the square root of that sum
PSD=<square_root>(A1AV+A2AV+A3AV) (The square root of the sum for AVs for ALL 3 activities forming the critical path in our example)

3. So the Project Duration Estimate Range is the expected project duration (the sum of EADs) plus or minus the project standard deviation (the square root of the sum of the activity variances).
Range=EPD+-PSD (for 68.27% confidence level), or
Range=EPD+-(PSD*2) (for 95.45% confidence level)

CPM (Critical Path Method):

As mentioned above, the CPM (Critical Path Method) is a technique we use AFTER we have found the duration of activities, that is AFTER applying PERT analysis, and involves determining the longest path through the network diagram (the critical path), the earliest and latest an activity can start, and the earliest and latest it can be completed.

We use this technique to help us keep our project on time and decide early whether we need to take any necessary proactive or corrective action so as to safeguard the project from undesirable developments.

Unfortunately, I can't describe the whole technique here but you can find all the answers you are looking for in this tutorial: http://headfirstlabs.com/PMP/criticalpath/HeadFirstPMP_CriticalPathDrill.pdf

I hope you find it useful.

Good luck!

Adapted from: Rita Mulcahy's PMP Exam Prep, 11th Edition | PMBOK Guide, 4th Edition
answered 8 years ago by anonymous edited 8 years ago by MaplePM

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