Float Calculation in Critical Path Method: Comprehensive Guide for PMI-SP Exams
Float Calculation in the Critical Path Method
Why Float Calculation Is Important
Float calculation is a crucial component of project scheduling and the Critical Path Method (CPM). It helps project managers to:
• Identify schedule flexibility
• Prioritize tasks effectively
• Allocate resources optimally
• Manage project risks proactively
• Make informed decisions about schedule compression
In the PMI-SP certification exam, float calculation questions test your ability to understand schedule flexibility and task dependencies - essential skills for schedule management professionals.
What Is Float in Project Management?
Float (also called slack) represents the amount of time a task can be delayed before it impacts another task or the project completion date. There are several types of float:
Total Float: The amount of time a task can be delayed before affecting the project end date.
Free Float: The amount of time a task can be delayed before affecting the start of any successor task.
Project Float: The amount of time the entire project can be delayed before missing its deadline.
Interfering Float: The difference between total float and free float for a task.
How Float Calculation Works
Step 1: Create the Network Diagram
Draw a network diagram showing all project activities and their dependencies.
Step 2: Forward Pass
Calculate Early Start (ES) and Early Finish (EF) dates for each activity:
• ES = Latest EF of all predecessor activities
• EF = ES + Duration - 1
Step 3: Backward Pass
Calculate Late Start (LS) and Late Finish (LF) dates for each activity:
• LF = Earliest LS of all successor activities
• LS = LF - Duration + 1
Step 4: Calculate Float
• Total Float = LS - ES (or LF - EF)
• Free Float = ES of successor - EF - 1
Key Formula: Total Float = Late Start - Early Start = Late Finish - Early Finish
Float Calculation Example
Consider a simple project with activities A, B, C, and D:
Activity A: Duration 3 days, no predecessors
Activity B: Duration 5 days, predecessor A
Activity C: Duration 4 days, predecessor A
Activity D: Duration 2 days, predecessors B and C
Forward Pass:
A: ES=1, EF=3
B: ES=4, EF=8
C: ES=4, EF=7
D: ES=9, EF=10
Backward Pass (project duration is 10 days):
D: LF=10, LS=9
B: LF=8, LS=4
C: LF=8, LS=5
A: LF=3, LS=1
Total Float:
A: LS-ES=1-1=0 (on critical path)
B: LS-ES=4-4=0 (on critical path)
C: LS-ES=5-4=1 (has 1 day of float)
D: LS-ES=9-9=0 (on critical path)
Exam Tips: Answering Questions on Float Calculation
1. Practice the formulas repeatedly. Make sure you can calculate ES, EF, LS, LF, and various types of float quickly and accurately.
2. Always draw the network diagram when solving float calculation problems, even if not required, to visualize relationships.
3. Double-check your calculations, especially when determining the critical path. The critical path consists of activities with zero total float.
4. Pay attention to calendar considerations in exam questions, such as working days vs. calendar days.
5. Watch for unusual dependencies like Finish-to-Finish or Start-to-Start, which change how you calculate float.
6. Identify trick questions where partial information is provided, requiring you to derive missing values.
7. Remember that activities on the critical path have zero total float, but not all activities with zero total float are necessarily on the critical path (in case of parallel critical paths).
8. For PDM (Precedence Diagramming Method) questions, remember to account for leads and lags when calculating float.
9. Focus on conceptual understanding, not just mechanical calculations. Understand what float means for project flexibility.
10. Be methodical - follow the same steps each time to minimize calculation errors under exam pressure.