Project Schedule Planning: PERT vs. CPM

Two of the most commonly used tools in project schedule planning are the “Project (or Program) Evaluation and Review Technique” (PERT) and the “Critical Path Method” (CPM).

But what are the differences between them?

PERT was developed as a project schedule planning technique in the 1950’s for the U.S. Navy Special Projects Office, while CPM was developed at roughly the same time by Morgan R. Walker and James E. Kelly for DuPont. Both methods are used to identify the minimum time needed to complete a project by considering all inter-dependant project activities that form the longest path or duration.

There are really only two fundamental differences between PERT and CPM, and these are:

• PERT applies an “Activity-on-Arrow” network diagram, whereas CPM applies an “Activity-on-Node” network diagram. Activity-on-Arrow means the network diagram depicts each milestone event as a node, and shows the activity information on the arrows joining each milestone event. Activity-on-Node shows the activity information as a node, and links one activity to the next, rather than linking one milestone to the next. The differences between the two schematic models is shown below.

• The second, and more important, difference is that traditional CPM applies a single duration and cost estimate for each activity, whereas traditional PERT applies a 3-point weighted average duration estimate (optimistic, most likely, and pessimistic) for each activity, and does not consider cost. The PERT weighted average duration is calculated as follows:

Te = (To + 4×Tm + Tp) ÷ 6              where:

Te = Expected Duration

To = Optimistic Duration

Tm = Most Likely Duration

Tp = Pessimistic Duration

These days, CPM and PERT have been largely absorbed into a single common technique which applies the preferred CPM “Activity-on-Node” diagrammatic model, and uses the PERT 3-point weighted average duration calculation.

Both PERT and CPM rely upon the analysis of four primary schedule components, being:

1. A list of all activities required to complete the project
2. The expected time that each activity will take to complete
3. The dependencies between each activity
4. Logical start and end points for each set of activities

Critical Path Analysis:

The critical path in any project is the longest path of inter-dependant activities required to achieve a logical end point. Critical Path Analysis takes into account the earliest and latest times that each activity can start and finish without making the project longer. In doing so, the analysis determines which activities are “critical” to the project schedule, in that all critical activities reside on the longest path from project start to project finish, and thereby define the minimum overall project duration.

Activities that can be delayed, or extended beyond their planned duration, without extending the overall duration of a project are considered to be non-critical activities that have “float” (also known as “slack”). An activity that can be delayed or extended without causing a delay in any subsequent activities is said to have “free float”, and an activity that can be delayed or extended without causing a delay to the overall project is said to have “total float”.

The amount of total float available to any activity is calculated in one of two ways: Either by subtracting the earliest start date from the latest start date, or by subtracting the earliest finish date from the latest finish date of the activity. Both methods will yield the same result, which will be the amount of time that the activity can be delayed without affecting the latest finish date of any subsequent activities.

The amount of free float available to any activity is calculated by subtracting the earliest finish date of the activity from the earliest start date of its nearest direct successor activity. This is the amount of time that the activity can be delayed without affecting the earliest start date of any subsequent activities. When an activity has zero total float, it will also have zero free float.

Critical Path Analysis is also used to calculate the “drag” on a project. In other words, the amount by which a project's duration is extended by each critical path activity. Drag is calculated by comparing critical path activity durations with each amount of total float in all other parallel activities. If a critical path activity has no other activities in parallel, its drag is equal to its duration. If a critical path activity has other activities in parallel, its drag is equal to whichever is less: its duration, or the total float of the parallel activity with the least amount of total float.

Schedule Risks:

A schedule is one of the project drivers that is most susceptible to risk. This is because schedules comprise multiple inter-dependant activities, each of which usually contain multiple uncertainties.

Consider, for example, one of the very first activities involved in the construction of a building, which is site excavation and preparation. This is a relatively straight forward process, but it is reliant on a number of factors in order for it to be completed in line with the planned project schedule. These include:

• Ensuring the required machinery, materials, utilities and other resources are all available for use on the planned start date, and remain so throughout the activity duration.
• Ensuring the required machinery, materials, utilities and other resources remain operational throughout the activity duration.
• Ensuring all site permits are in place and valid throughout the activity duration.
• Ensuring the machinery, tools and equipment can cope with site conditions.
• Ensuring unexpected weather does not affect progress.
• Ensuring unexpected labour issues do not affect progress.
• Ensuring unexpected health & safety issues do not affect progress.
• Ensuring unexpected security issues do not affect progress.
• Ensuring unexpected regulatory issues do not affect progress.

Most construction companies are well versed in the management of these types of risks, and will plan for them as a matter of course. The point is merely to highlight the number of risk factors, and related uncertainties, associated with even the most basic of project activities. Add to that the number of different activities required to complete a typical project, along with all their inter-dependencies, and the level of uncertainty in a project schedule can grow exponentially.

One possible solution to maximise schedule robustness is to include a safety buffer in the baseline schedule to absorb any anticipated disruptions. This is called proactive scheduling. However, pure proactive scheduling is not a realistic option. Incorporating sufficient safety in a baseline schedule, which allows for every possible disruption, would undoubtedly lead to an unacceptably long schedule. A second approach, termed reactive scheduling, consists of defining a procedure to react to a range of disruptions that cannot be absorbed by the baseline schedule.

At the heart of schedule risk is the critical path, as this is the longest activity path which defines the minimum project duration and contains the least amount of total float. Any delay to activities on the critical path which have zero total float will delay the overall project schedule. It is therefore crucial to protect the critical path as much as possible, and the most effective way to do this is to ensure that the planned duration of each activity is as accurate and robust as possible.

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