Backward Pass

 

Key Points:

1. Backward Pass Overview:

   • The backward pass calculates the Late Finish (LF) and Late Start (LS) for each activity in a project network diagram.
   • LF: The latest an activity can finish without delaying the project.
   • LS: The latest an activity can start without delaying the project.

2. Backward Pass Steps:

   • Start at the end of the diagram (last activity).
   • Begin with the critical path duration (e.g., 11 in this example).
   • Use the formula:
     LS = LF - Duration + 1.

3. Example Backward Pass Calculation:

   • Activity E:
     • LF = 11 (critical path duration), Duration = 2.
     • LS = 11 - 2 + 1 = 10.
   • Activity C:
     • LF = 10, Duration = 4.
     • LS = 10 - 4 + 1 = 7.
   • Activity D:
     • LF = 9 (smallest LF of converging paths to Activity B), Duration = 3.
     • LS = 9 - 3 + 1 = 7.
   • Activity B:
     • LF = 7 (smallest LF of converging paths), Duration = 2.
     • LS = 7 - 2 + 1 = 6.
   • Activity A:
     • LF = 4, Duration = 3.
     • LS = 4 - 3 + 1 = 2.

4. Converging Paths in the Backward Pass:

   • At path convergence points, select the smallest LF among the converging paths for the backward calculation.
   • This ensures that the subsequent activities start as late as possible without delaying the project.

5. Critical Path and Slack:

   • Activities on the critical path have no slack (Slack = 0).
     • LF = EF and LS = ES for critical path activities.
   • Non-critical path activities (e.g., Activity D) have slack, calculated as:
     • Slack = LF - EF or Slack = LS - ES.

6. Practical Example of Slack:

   • Activity D:
     • LF = 9, EF = 8, LS = 7, ES = 6.
     • Slack = 9 - 8 = 1 (or 7 - 6 = 1).

7. Backward Pass Tips:

   • When going backward:
     • Use the largest number at convergence points for the forward pass.
     • Use the smallest number at divergence points for the backward pass.
   • Focus on calculating one step at a time, ignoring unrelated numbers until needed.

8. Outcome of the Backward Pass:

   • The diagram now contains all four values for each activity: ES, EF, LS, LF.
   • Identifies:
     • Total project duration.
     • Critical path activities (no slack).
     • Non-critical activities with slack.

9. Encouragement and Next Steps:

   • If the backward pass is unclear, re-watch the video and practice with simpler and more complex examples.
   • The backward pass is as manageable as the forward pass when broken into logical steps.

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Summary:

The backward pass determines the Late Finish (LF) and Late Start (LS) for activities in a project network diagram by calculating backward from the project’s end. The process identifies critical path activities with no slack and non-critical activities with slack, which can be delayed without affecting the project. The backward pass uses the formula LS = LF - Duration + 1 and requires selecting the smallest LF at convergence points. By completing the forward and backward passes, the network diagram provides a complete schedule, highlighting the critical path and slack for effective project management.