Backward Pass – A critical path method (CPM) technique for calculating the late start and late finish dates by working backwards through the schedule network from the project end date.
Key Characteristics:
- Reverse calculation: Works from project finish date back to project start
- Late dates: Calculates Late Finish (LF) and Late Start (LS) dates
- Critical path analysis: Essential component of CPM scheduling
- Float calculation: Used with forward pass results to determine total float
- Constraint-driven: Based on project completion requirements
Calculation Process:
- Start at project end: Begin with the project’s required completion date
- Work backwards: Move from successor to predecessor activities
- Calculate Late Finish (LF): Latest date an activity can finish without delaying project
- Calculate Late Start (LS): LF minus activity duration
- Continue upstream: Repeat for all activities in the network
Formulas:
- Late Start (LS) = Late Finish (LF) – Duration
- Late Finish (LF) = Earliest Late Start of all successor activities
Rules for Multiple Predecessors/Successors:
- Multiple successors: LF = earliest LS of all immediate successors
- Multiple predecessors: Each gets the same LF, then calculate individual LS
- Network end: Final activities use project completion date as LF
Purpose and Benefits:
- Float calculation: Enables determination of schedule flexibility
- Critical path identification: Activities with zero float are on critical path
- Schedule optimization: Identifies where delays can be absorbed
- Resource planning: Helps determine latest possible start dates
- Risk management: Shows schedule buffer availability
Example:
If Activity B must finish by Day 10 (LF = 10) and has a duration of 3 days:
- Late Start (LS) = 10 – 3 = Day 7
Integration with Forward Pass:
- Forward Pass: Calculates Early Start (ES) and Early Finish (EF)
- Backward Pass: Calculates Late Start (LS) and Late Finish (LF)
- Total Float: LS – ES or LF – EF
- Critical Path: Activities where ES = LS and EF = LF (zero float)
Dependencies Handling:
- Finish-to-Start (FS): Most common, successor LS = predecessor LF
- Start-to-Start (SS): Adjusted for relationship lag/lead
- Finish-to-Finish (FF): Both activities must finish by same constraint
- Start-to-Finish (SF): Rare relationship requiring special calculation
Common Applications:
- Schedule development: Creating realistic project timelines
- What-if analysis: Testing different completion dates
- Resource leveling: Understanding scheduling flexibility
- Change impact: Assessing effects of scope or duration changes
- Performance monitoring: Comparing actual vs. planned progress
Related Terms:
- Forward Pass: Calculates early start and finish dates
- Critical Path: Longest path through the project network
- Total Float: Amount of delay an activity can absorb
- Late Start (LS): Latest date an activity can start
- Late Finish (LF): Latest date an activity can finish
- Schedule Network Diagram: Visual representation of activity relationships
Tools and Techniques:
- Manual calculation on network diagrams
- Project management software (MS Project, Primavera)
- Spreadsheet applications
- Specialized scheduling tools
