Understanding the concept of float in CPM scheduling is foundational to achieving data-driven project controls. Float, also known as “slack,” is the amount of time an activity–or series of activities–can be delayed without impacting the project’s end date. Think of it as how much wiggle room you have—which is quite important information to know when managing a commercial construction project.
Types of Float in CPM
- Total Float: The amount of time you can delay an activity before impacting the project’s end date.
- Free Float: The amount of time you can delay an activity without impacting subsequent activities.
As the Project Management Institute (PMI) points out, once float is calculated, a string of critical activities emerges. This sequence, known as the critical path, has the smallest amount of float and represents the project’s total duration. The term “critical” underscores the minimal float in these activities– precisely, zero.
Float in CPM as an Indicator of Resource Priority
Differentiating between an activity with two months of float and one with zero is extremely important for managing resources. Obviously, everyone sees a red path surrounded by green and blue activities. However, float indicates an activity’s criticality, making it also an indicator of how to prioritize resources.
Critical Path
Think of the critical path, highlighted in red, as your high-priority task list. These are the activities where prioritizing your resources and efforts is most important to meet your project’s deadlines. If any of these tasks are delayed and no acceleration happens to make up for it, the entire project will be delayed.
Near-Critical Path
Activities closely following the critical path are known as the near-critical path. These activities may not be on the critical path physically. However, they are still important because they have minimal float – usually around ten days on average, though this can vary depending on project size and complexity.
Importance of Keeping an Eye on Near-Critical Activities
Activities with a total float value of less than 10 days should be considered near-critical. They require attention because they can quickly become critical if delayed. In particular, activities with less than five days of float (3-5 days) should be treated with similar urgency as those on the critical path.
Practical Scenario
Consider a project where your critical path consists of foundation work, framing, and roofing. These tasks are non-negotiable in their timelines. Meanwhile, tasks like electrical wiring or plumbing may have a bit more float but are near-critical. If the plumbing task has only four days of float, a delay here could quickly affect the critical activities that follow, such as installing fixtures or inspections.
How to Analyze Float in CPM
Relying on float values to make decisions is a proven tactic for resource management. However, scheduling has many pitfalls if you are unaware of best practices.
When reviewing a schedule, it’s important to analyze:
- Average Float: Determine the average amount of float across all activities.
- High Float Activities: Identify how many activities have more than two months of float (e.g., 44 days).
If you find that a large percentage (over 50%) of activities have more than two months of float, it might indicate missing logic in the schedule.
Missing Logic
The most important rule in CPM scheduling is that every activity, except the first in last, must have a predecessor and successor. This is important because, without logic, float opens up.
Typically, the more float opens, the more compressed the schedule becomes. By design, activities start stacking on their own, resulting in an erroneous critical path. While hard logic is usually present within the schedule, crew logic is often missing, causing timelines to become compressed and unrealistic.
What is Missing Crew Logic?
Crew logic is a form of “soft logic” that assigns relationships to activities that reflect where a crew will move next based on means, methods, and constraints. It is responsible for representing the flow of work in your schedule.
Float as an Indicator of Missing Crew Logic:
Excessively high float across many activities suggests missing crew logic.
Additionally, if the critical path flows straight through the project without considering the trades involved, it is missing crew logic. For example, if you were building a high-rise with 80 floors and only the top floor was deemed critical but doesn’t account for the trades working on the intermediate floors, there would likely be an issue.
Constraints and Their Impact on Float in CPM
Placing constraints on activities, particularly finish constraints, can complicate your schedule analysis.
- Constraints can turn activities with float values between 0 and 30 days red, making them seem critical.
- The critical path may no longer reflect the true driving factors of the project.
Determining Acceptable Values of Float in CPM
For linear projects, I recommend minimizing float and lengthening the critical path to ensure steady project progression. On the other hand, for both horizontal and vertical projects, it’s vital to maintain a reasonable amount for all parties. A strategy to achieve this is by integrating crew logic into the scheduling process, which ensures that labor constraints are taken into account.
The goal is often to have a month or less of float in CPM. Shorter float durations allow for clearer visibility into when and where impacts happen. Having a tight schedule, meaning one that thinks through all necessary logic, allows for clear identification of the critical path that drives the project. This kind of schedule control ensures any impacts to the schedule can be identified and addressed promptly.
1. Analyze Project Complexity
Complex projects typically need more wiggle room to account for uncertainties. Assess the complexity by considering the following:
- The number of activities and their interdependencies
- Involvement of multiple trades and subcontractors
- Duration and scope of the project.
2. Evaluate Historical Data
Review float values from similar projects in the past to identify patterns. Look at:
- Average float durations that led to successful project completions
- Float durations in projects that experienced delays or issues.
3. Assess Risk Factors
Consider potential risks that could impact your schedule, such as weather, materials, workforce, and approvals.
4. Use Industry Standards
Refer to industry benchmarks for float levels. SmartPM recommends:
- Activities with less than 10 days of float are considered near-critical
- Activities with 2-5 days of float should be treated as critical.
- A total float value of over 44 days presents a significant risk to schedule quality.
- Excessive high-float activities (anything over two months) indicate flawed logic.
5. Implement Contingency Planning
Establish a buffer for high-risk activities. This might involve:
- Allocating additional float to tasks with high uncertainty.
- Scheduling buffer tasks between critical milestones to absorb delays.
6. Monitor and Adjust During Execution
Regularly update your schedule and adjust float based on project progress:
- Use tools like SmartPM that automatically track changes and their impact on float.
- Reallocate resources to manage tasks with diminishing float.
7. Define Float Guidelines
Set internal guidelines for acceptable float levels. For example:
- Aim for an average float of 20-30% of total project duration.
- No more than 20% of activities should be near-critical.
In essence, float can offer a buffer against unexpected changes, allowing activities to be postponed without impacting the overall end date.
Slacking Off on Slack
Float is only as valuable as the CPM schedule; this comes with building your schedules with best practices and ensuring these best practices are maintained throughout the updating process.
So, number one, ensure you have high schedule quality. In fact, several DCMA schedule quality checks are associated with float, including missing logic, high-float activities, and average activity total float.
Schedule quality control ensures schedules are built with best practices, allowing project teams to trust what their schedules are telling them. SmartPM streamlines the process of schedule quality control from baseline and throughout the updating process. If you’d like to learn more about how SmartPM can help you achieve smarter project management, request a demo, and we’d be happy to show you how.
