Float, also known as slack, is a key concept in schedule network analysis that refers to the amount of time an activity can be delayed without causing a delay to subsequent activities or the overall project completion date. Calculating float is crucial for understanding the flexibility within a pro…Float, also known as slack, is a key concept in schedule network analysis that refers to the amount of time an activity can be delayed without causing a delay to subsequent activities or the overall project completion date. Calculating float is crucial for understanding the flexibility within a project schedule and for identifying which activities have scheduling leeway and which are time-critical.
There are two main types of float: Total Float and Free Float. Total Float is the difference between the late finish and early finish of an activity (or between the late start and early start). It represents the total time an activity can be delayed without delaying the project's end date. Free Float, on the other hand, is the amount of time an activity can be delayed without delaying the earliest start date of any successor activities.
Calculating float involves performing forward and backward pass calculations through the project network diagram. The forward pass determines the earliest possible start and finish times for each activity, while the backward pass calculates the latest possible start and finish times without delaying the project. The differences between these times provide the float values.
Understanding float helps project managers in several ways. By identifying activities with high float, resources can be reallocated from less critical tasks to those on the critical path if needed to optimize efficiency. Float analysis also aids in risk management by highlighting potential areas where delays can be absorbed without affecting the project's overall timeline. Moreover, it allows for better planning of resource availability and scheduling flexibility.
Float calculations are integral for schedule optimization and for making informed decisions when changes occur during project execution. They enable proactive adjustments to the schedule in response to unforeseen delays or opportunities to accelerate the project. In essence, float analysis provides a buffer that contributes to effective time management and helps ensure the successful delivery of the project within the desired timeframe.
Float Calculation in Project Management
Understanding Float Calculation in Project Management
Float (also called slack) is a critical concept in project schedule management that represents the amount of time an activity can be delayed without delaying the project completion date or violating a schedule constraint. Mastering float calculation is essential for the PMI-SP certification and effective project schedule management.
1. Total Float (TF): The amount of time an activity can be delayed from its early start date before it delays the project end date.
Formula: TF = Late Finish (LF) - Early Finish (EF) or TF = Late Start (LS) - Early Start (ES)
2. Free Float (FF): The amount of time an activity can be delayed before it delays the early start of any successor activity.
Formula: FF = Early Start of successor - Early Finish of activity
3. Project Float: The amount of time a project can be delayed beyond its planned completion date before it exceeds constraints like contract dates.
How Float Calculation Works
Step 1: Create the Network Diagram • Identify all activities • Establish dependencies • Determine durations • Draw the network diagram
Step 2: Forward Pass (Calculate Early Times) • Start with ES = 0 for initial activities • Calculate EF = ES + Duration - 1 • For subsequent activities, ES = largest EF of predecessors + 1 • Continue until project end
Step 3: Backward Pass (Calculate Late Times) • For the last activity, LS = ES and LF = EF • For preceding activities, LF = smallest LS of successors - 1 • Calculate LS = LF - Duration + 1 • Continue until project start
Step 4: Calculate Float • Total Float (TF) = LF - EF or LS - ES • Free Float (FF) = ES of successor - EF of activity
Example Calculation
Consider a simple network with 3 activities:
Activity A: Duration = 5 days Activity B: Duration = 3 days (depends on A) Activity C: Duration = 4 days (depends on A)
Forward Pass: A: ES = 1, EF = 5 B: ES = 6, EF = 8 C: ES = 6, EF = 9
Backward Pass: C: LF = 9, LS = 6 B: LF = 9, LS = 7 A: LF = 5, LS = 1
The critical path consists of activities with zero float. These activities determine the project duration, and any delay in a critical activity will delay the entire project.
In our example, Activities A and C form the critical path.
Exam Tips: Answering Questions on Float Calculation
• Read carefully: Pay attention to whether the question asks for total float, free float, or project float.
• Draw it out: For complex networks, always draw the diagram to visualize dependencies.
• Check your calculations twice: Common errors occur during the forward and backward pass calculations.
• Remember the formulas: Memorize the key formulas for total float and free float.
• Watch for calendar adjustments: Some exam questions may involve non-working days or calendar constraints.
• Consider resource constraints: Some questions may factor in resource limitations affecting float.
• Start with zero: Remember that in PDM (Precedence Diagramming Method), day one is actually day zero in calculations.
• Practice with examples: Work through sample problems to build speed and accuracy.
• Keep track of units: Make sure you're using consistent time units (days, hours, etc.).
• Look for trick questions: Sometimes the exam will test your understanding by presenting unusual network constraints.
Mastering float calculation requires practice. Work through multiple examples until the process becomes second nature. Understanding float is key to becoming an effective project schedule manager and passing the PMI-SP certification exam.
PMI-SP - Float (Slack) Calculation Example Questions
Test your knowledge of Float (Slack) Calculation
Question 1
In a construction project, the project manager wants to calculate the Free Float of Activity G. The data given are: Activity G - ES (Early Start) = 11, EF (Early Finish) = 19; Activity H - ES (Early Start) = 22. What is the Free Float of Activity G?
Question 2
A project manager is reviewing the critical path of a project. They discovered that the project is running late and need to determine the Total Float of Activity E to identify the impact. What is the Total Float of Activity E considering the following data? Activity E: ES (Early Start) = 4, EF (Early Finish) = 10, LS (Late Start) = 9, LF (Late Finish) = 15.
Question 3
In calculating float values, what statement best describes Total Float in relation to Free Float?
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