Start Day Project: Forward and Backward Pass Calculations
PMBOK v8 Definition
The forward pass begins at the project start and calculates the earliest possible start and finish dates for each activity. The backward pass begins at the project end and determines the latest allowable start and finish dates without delaying the project. The Critical Path Method (CPM) involves schedule network analysis by calculating early start (ES), early finish (EF), late start (LS), and late finish (LF) dates for each activity, without regard to resource limitations, by performing a forward and backward pass through the schedule network.
Why It Matters for the Exam
This concept appears in approximately 15-20% of schedule management questions on the PMI exam. Questions frequently test your ability to calculate float, identify the critical path, and understand the relationship between forward/backward pass calculations. Expect scenario-based questions where you must determine which activities have zero total float or calculate the minimum project duration.
Key Points to Remember (for the exam)
- Forward Pass Formula: EF = ES + Duration – 1 (when project starts on Day 1)
- Backward Pass Formula: LS = LF – Duration + 1 (when project starts on Day 1)
- Total Float Calculation: Total Float = LS – ES or LF – EF
- Critical Path Definition: Activities with zero total float form the critical path
- Critical Path Identification: The longest path through the schedule network determines the shortest possible project duration
- Key Output: Early and late dates define time windows, not the actual schedule
- Resource Exclusion: CPM calculations are performed without regard to resource limitations
Typical PMI Exam Example
A project network diagram shows Activity A (duration 3 days) with ES = Day 1 and EF = Day 3. Activity B follows A with duration 4 days. If the backward pass shows LF = Day 10 for Activity B, what is the LS for Activity B?
Answer: LS = LF – Duration + 1 = 10 – 4 + 1 = Day 7
PMI Exam Traps
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Trap: Confusing forward pass with backward pass calculations
- Reality: Forward pass calculates earliest dates (start at project beginning); backward pass calculates latest dates (start at project end)
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Trap: Using EF = ES + Duration instead of EF = ES + Duration – 1
- Reality: When project starts on Day 1, subtract 1 to account for the start day being counted
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Trap: Thinking critical path is always the shortest path
- Reality: Critical path is the longest path and determines the shortest possible project duration
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Trap: Assuming activities on the critical path cannot have float
- Reality: Critical path activities have zero total float; non-critical activities may have positive float
Important PMI Connections
| Related Concept | Relationship Type | Exam Attention Point |
|---|---|---|
| Schedule Network Diagram | Input to CPM | Network logic determines activity sequencing for pass calculations |
| Activity Duration Estimates | Input to CPM | Duration values are used in ES/EF/LS/LF calculations |
| Leads and Lags | Modifier of CPM | Can adjust start/finish dates and affect float calculations |
| Schedule Compression | Output of CPM | Critical path identification enables focused crashing or fast-tracking |
Quick Review Questions
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If Activity X has ES = Day 5 and duration = 6 days, what is its EF when the project starts on Day 1?
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An activity has LF = Day 20 and LS = Day 15. What is its total float?
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Which sequence of activities determines the shortest possible project completion time?
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What is the difference between early start dates calculated by forward pass and late start dates calculated by backward pass?
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Why are resource limitations excluded from CPM calculations?
PMBOK v8 Reference
Section 5.4 - Critical Path Method (CPM)
Note: The PMBOK v8 context provided uses the accepted convention of the project starting on Day 1 for calculating start and finish dates. Other accepted conventions may be used, but this is the standard tested on the PMI exam.