CPM Scheduling: The basics

Critical Path Method (CPM) is a project modeling and planning technique that the military and major US companies developed to manage the manufacturing processes of sophisticated weaponry.  Now, CPM is used in all forms of projects.  Any project with interdependent activities can apply this method of mathematical analysis. 

In construction, a CPM schedule has become as meaningful a tool to a project manager or superintendent as a hammer is to a carpenter or a wrench is to a mechanic. It helps a diligent contractor accurately figure out how long a complex project will take and whether the project is ahead or behind schedule.

Here are the basics:

A CPM model starts with the following input:

1.   A list of all activities required to complete the project

Different arrays of activities define a construction project’s scope. For example, the four walls of a barn could be considered a single activity or could be broken down into north, east, south and west walls and, even further, into framing, sheathing and siding.  The degree of definition is sometimes specified in the contract but is generally determined by the author of the schedule.  When a project is linear, like miles of tunnel, the number of activities tends to be smaller than a complicated building.   

Filters are used to select certain activities that relate to imminent work (short interval schedules) or the work of specific subcontractors.

2.   Duration: The time that each activity will take to complete

The more resources a contractor has at his or her disposal, the faster a task can be completed.  For example, if a contractor has two workers to frame the walls of a shed, it may take a week.  However, if 10 workers are available, the same task may be completed in a day.

3.   Logic: The relationships or dependencies between the activities

The relationships between activities in a CPM schedule are collectively called “logic”.  There are different relationships such as

Finish-start: In the most frequently used relationship, the successor activity cannot start until its predecessor has finished.  This relationship would be appropriate for “framing walls” (the successor) that cannot commence until the “Foundation walls” (the predecessor) is complete.

Start-start: A “start-start” relationship is used for separate activities that can commence concurrently. An example might be “Submittals: Shop drawings for Concrete Rebar” and “Mobilize Field Office”.  In this case, the start dates of the concurrent activities might both be tied to completion of a mutual predecessor or, the start of one of the concurrent activities may be tied to the start of the other concurrent activity. 

Lag represents a delay either with starting a successor after the predecessor is completed or a delay starting one activity after the start of a concurrent activity.  For example, curing concrete may be handled as a lag after pouring a concrete foundation and before framing walls on the concrete foundation.  Another start-start lag might be used to delay the start of installing siding until some period of time after windows have been installed.

Float:

Total float is the amount of time that the start of a scheduled activity can be delayed from its early start date without delaying the project finish date or violating a schedule constraint.

Free float is the amount of time that the start of a scheduled activity can be delayed from its early start date without delaying the early start date of any immediately following schedule activities.  

Critical path: Using activities, durations and logic, CPM scheduling software mathematically calculates the longest path of planned activities from the beginning to the end of the project, and the earliest and latest dates that each activity can start and finish without increasing overall duration.  This process identifies which activities are "critical" (part of the longest path) and which others have float. 

In project management, a “critical path” is the sequence of project activities that combine to produce the longest overall duration.  This determines the shortest time possible to complete the project.  Because there is no float associated with activities on the critical path, any delay or extended duration of these activities directly impacts the planned project completion date. 

Negative Float: When a project falls behind schedule, but its completion date is constrained (and, therefore, cannot slip) you have negative float.  Here, a path in a network becomes hypercritical with activities on that path having a float of less than zero.  The negative float indicates the amount of time that must be picked up in order to achieve an imposed date or “constraint”.  The schedule can reflect a delay to a critical path activity with either negative float to affected activities (if there is a constraint preventing the critical path from extending into the future), and/or slippage of the completion date. The total delay experienced by the project is the sum of the negative float and slippage of the completion date.

Hammocks: A hammock is a task without a fixed time duration.  Instead, it automatically calculates its duration from the tasks that it is associated with in terms of resources and/or cost. For example, a hammock called “rent backhoe” is created for the rental of a backhoe costing $200 per day.  “Rent backhoe” has a start-to-start relationship with the first task that requires the backhoe, and a finish-to-finish relationship with the last task that requires the backhoe. If the hammock or string of tasks using the backhoe is 20 days, then the rental cost is automatically calculated as $4,000.  If the schedule is compressed to reduce that span to 15 days, then the backhoe cost is automatically recalculated to $3,000.  By using hammocks, a project manager can see the financial implications of squeezing time out of the schedule. 

CPM calculations allow the contractor to prioritize activities and to shorten the planned critical path of a project by pruning critical path activities, "fast tracking" (i.e., performing more activities in parallel), and/or by "crashing the critical path" (i.e., shortening critical path activity durations by adding resources or working overtime).  Resource leveling applies resources to activities to optimize manpower and cost data produces a schedule of values and cash flow projections.

Progressing a schedule is the customary regular and routine updating of the percent complete of each current activity.  This is usually done on a monthly basis to coincide with the contractor’s billing cycle. 

A contractor can be evaluated by reviewing his or her CPM schedules for size, complexity, realism and whether or not it is “progressed”. 

A CPM schedule is often not realized precisely, as estimates are used to calculate activity durations based on assumed resource levels subject to availability: if one change is made, the results of the analysis may change.  This could cause an upset in the implementation of a project if estimates are blindly believed, and if changes are not addressed promptly. However, the structure of CPM analysis is such that variance from the original schedule can be measured, and its impact mitigated.

An important element of project postmortem analysis is the As-Built Critical Path (ABCP), which analyzes the specific causes and impacts of changes between the planned schedule and eventual schedule as actually implemented.  If a CPM is regularly and routinely progressed with “actual” activity start and finish dates, an ABCP automatically results at the end of the project.

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