Backward Pass
A CPM technique that figures out each activity's late start and late finish by moving backward through the schedule from the project's completion date.
Key Points
- Part of the critical path method, performed after or alongside the forward pass.
- Starts at the project finish (or latest imposed finish) and works backward through the network.
- For each activity: late finish (LF) is typically the minimum late start (LS) of its successors; late start (LS) = LF - duration, adjusted for leads/lags and relationship types.
- Provides late dates used to compute float and confirm the critical path (zero total float).
Example
You have a project that must finish on day 50. Activity D (3 days) feeds two successors: E and F. From earlier calculations, LS(E) = day 30 and LS(F) = day 32. In the backward pass, D's LF = min(30, 32) = 30, and D's LS = 30 - 3 = day 27. Repeat this for all predecessors, moving backward until you reach the project's start.
PMP Example Question
Which scheduling step uses the project finish date to determine activities' late start and late finish dates?
- Forward pass
- Backward pass
- Resource leveling
- Rolling wave planning
Correct Answer: B — Backward pass
Explanation: The backward pass starts from the project completion date and calculates late dates for each activity, enabling float and critical path determination.
