The PERT Analysis – Part 2

In the previous post, we began a formal discussion on PERT analysis and how it is used as a project estimation and flow technique. (Note: if you have not read the previous post, please access it at the following link: The PERT Analysis – Part 1)

As a brief refresher, the idea behind a PERT analysis is twofold:

1) It gives an overview of the various constituent pieces that are part of a given program and shows (graphically) how they relate to one another.

2) The PERT also yields the ‘critical path‘ of the program, which is a mathematical summation of the various paths in the PERT diagram which eventually provides the path with the longest duration. Which in turn becomes the critical path.

One of the main formulas utilized when calculating various completion estimates for tasks that are part of the overall program is the following:

TE = (O + 4M + P) ÷ 6

The variables listed in the formula are as follows:

TE – Time Estimate

O – Optimistic Time

M – Most Likely Time

P – Pessimistic Time

Think of this as an ‘over/under’ technique, whereby the individuals working on the tasks provide three timeline estimates denoting their most optimistic, pessimistic and likely timeframes for the task. Those estimates are then plugged into the above formula to come up with the time estimate for the task. Note that the usage of this formula is not mandatory for PERT and that single estimates can also be leveraged. The most important consideration is that each task (activity) have some estimate associated with it.

To add further granularity to the overall PERT schedule, various estimates on earliest and latest start or finish times for each activity can be calculated. Note that each activity can have all of these estimates associated with it. In order to determine Early Start, Early Finish, Late Start and Late Finish for each activity, one must examine each node and ascertain the values relating to the predecessor nodes. So for example, if the activity between Node 1 and Node 2 is deemed to be 2 weeks, then the Early Start for Node 2 is 2 weeks from the given date. (Assuming Node 1 is the start node)  Leveraging these calculations for each node can give both optimistic and pessimistic timescales to each activity as well as take into account potential slippage events from predecessor activities.

Some additional definitions worth mentioning relating to PERT are as follows:

Slack (or Float)

This is the amount of time that a project or task can be delayed without causing slippage to subsequent tasks and overall project completion. Note that if a task is on the critical path, it cannot have a float associated with it since any delay to activities in the critical path will delay dependent activities further downstream. Positive slack denotes the project being ahead of schedule, negative slack indicates the project is behind schedule while zero slack indicates it is on schedule.

Fast Tracking

This is a technique whereby the most critical activities are performed in parallel. This is a concept that the project manager must engineer up front and should not be attempted once well into the project. The idea is that if critical activities are performed in parallel as opposed to sequentially, the likelihood of one activity causing slippage to subsequent activities is reduced. Note that care must be taken when deriving this solution. Dependencies will exist for various tasks and performing them in sequence may be the only viable option. (Eg. needing to build the foundation of a house before working on its frame) But if performed correctly, fast tracking can streamline a project and give more lead time to any issues that may arise without having to potentially delay subsequent tasks.

Crashing the Critical Path

In cases where the critical path cannot be effectively re-engineered for more parallel development, the project manager may instead decide to add more resources to items in the critical thereby reducing the amount of time for each one. This technique is referred to as ‘crashing the critical path’ and can be leveraged in cases where it is advantageous to utilize resources early in the project cycle to ensure completion of critical activities. (This is also referred to as ‘front loading’ in some cases) Note that this technique is often best performed when new resources have been added to a project, although it can also be achieved through effective re-distribution of existing resources.

SUMMARY

The PERT analysis can be an effective means by which to provide a good overview and timeline for a given project or program. As alluded to in the previous post, PERT can have some drawbacks if the chart has so many tasks and dependencies that it becomes unwieldy. In those cases, segmentation of the PERT into more cohesive pieces can assist. It may be possible to create a ‘master PERT’ that serves as a full program overview while smaller PERTs will denote the main portions of the primary PERT.

For further information on PERT and critical path analysis, please consult the following Wikipedia article for additional detail:

http://en.wikipedia.org/wiki/Pert

The PERT Analysis – Part 1

For every project, there are tasks that are outlined which serve as an indication of what needs to be completed and in what order to ensure the proper outcome and closure of the project. This can be expanded upon further in the concept of a program, which is a group of inter-related projects.

As a program and its constituent projects become more complex, inter-dependencies of tasks and project deliverables become an important piece of criteria to track. Successful completion of a project or program that has dependencies is a challenging endeavor. Care and diligence on the part of the project manager is paramount to ensure that specific dependencies are resourced and completed accordingly to ensure that other pieces of the overall project or program and also be initiated and eventually completed.

When it comes to properly cataloging and tracking project/program flow and interdependencies, it becomes imperative to do so effectively. With that being said, what is the best tool in the project manager’s arsenal to accomplish this task?

Program Evaluation and Review Technique

The Program Evaluation and Review Technique, or ‘PERT‘ for short, is a statistical tool and technique that is specifically designed to define, analyze, track and effectively represent the various constituent pieces of a project or program. It is mostly commonly used in conjunction with the critical path method (CPM) which is a technique used to schedule a set of project activities.

The main criteria for performing an effective PERT is to have the tasks, projects, interdependencies and timeframes all catalogued. Normally, this process will occur early in the project plan, regardless of inherent project process. The Work Breakdown Structure will generally drill down the project or program into its constituent task level activities. From there, each task can be given a timeframe for completion. (Note: for information on how to perform project scheduling and timeframe calculations, please consult the post: Project Scheduling-Tools & Techniques)

Once the various pieces are itemized, the PERT structure can be formed. The following are terminology definitions of the various pieces of the PERT that are used to produce the eventual visual chart are as follows:

PERT Event – Signifies the start or completion of an activity (task)

Predecessor Event – Precedes a given event but has no intervening events

Successor Event – Follows a given event but has no intervening events

A few key points in regards to the above definitions. The standard PERT Event consumes no resources and takes no time; it is merely used to signify the node of completion for various tasks. With regards to the predecessor and successor events, note that they can have any number of predecessors or successors within the entire structure.

Once the key attributes of the PERT analysis are itemized and catalogued, an actual dependency chart can be drafted to signify the overall flow of the project or program. As an example, consider the following:

A project has six activities, labelled A, B, C, D, E and F. Each activity has been given the following timelines for completion:

A=3wk

B=4wk

C=1wk

D=2wk

E=2wk

F=3wk

Each of the activities have varying dependencies that are displayed graphically as follows:

Once the activities are accurately drafted, with their inherent completion times and dependencies provided, the overall graph yields the estimated time for completion of the project. This estimate is referred to as the ‘critical path‘ and it denotes the longest path and timeframe based on the chart. In order to determine the critical path, all paths from the chart are evaluated with their completion times added together. So as per the above example, the paths and their mathematical sums are as follows:

A->D = (3+2)= 5wks

B->E = (4+2) = 6wks

A->C->F = (3+1+3) = 7wks

Thus, the longest timeframe or ‘critical path’ is A->C->F which is 7wks. Which denotes the overall time to complete the project.

Conclusions

The PERT method is an excellent mechanism to graphically display the primary project dependencies and overall flow. It is a high level overview that gives a good summation of project estimated completion time and overall variance. While PERT is valuable in demonstrating specific dependencies and overall project flow, it may become cumbersome if there are copious interdependencies in the overall project or program. In those cases, it may be advantageous to create multiple PERT charts denoting various sections of the project and then have those critical path estimates serve up to a global PERT that demonstrates the overall project or program flow and completion.

In the next post, we will delve into PERT and critical path further by discussing floats, fast tracking and earliest/latest start and finish times.

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