Step 1: List and Sequence: Identify every individual task, its estimated duration, and its immediate predecessors to establish the logical flow.
Step 2: Construct the Network: Draw nodes and arrows to represent the dependencies, ensuring that no activity can start until all preceding activities entering its start node are complete.
Step 3: Perform the Forward Pass: Calculate the EST for every node starting from zero; if multiple activities lead into a node, choose the largest resulting time value.
Step 4: Perform the Backward Pass: Set the LFT of the final node equal to its EST, then work backward to calculate LFTs for all other nodes, choosing the smallest value when multiple paths converge.
Step 5: Identify the Critical Path: Locate the sequence of nodes where and mark the connecting activities (often with double lines) to show they have zero float.
| Feature | Critical Activity | Non-Critical Activity |
|---|---|---|
| Float Time | Zero float; no delay possible | Positive float; some delay is acceptable |
| Impact of Delay | Delays the entire project completion | Can be delayed without affecting the end date |
| Management Focus | High priority for resource allocation | Flexible; resources can be 'borrowed' from here |
| Path Location | Located on the longest time path | Located on shorter parallel paths |
The 'Max/Min' Rule: Always remember that during the forward pass you take the maximum value at a junction, but during the backward pass you take the minimum value.
Verification of the Start: The LFT of the very first node (Node 1) must always calculate back to zero; if it does not, there is a mathematical error in your backward pass.
Double-Line Notation: In exams, the critical path is traditionally indicated by drawing two short parallel lines across the activity arrows to distinguish them clearly from non-critical tasks.
Sanity Check: Ensure the critical path is a continuous sequence from the first node to the last node; a 'broken' critical path usually indicates a logic error in the network construction.
Shortest vs. Longest: A common mistake is thinking the 'shortest' path on the diagram is the critical path; in CPA, the path with the most time is the one that limits the project speed.
Ignoring Dependencies: Students often forget that an activity cannot start until all incoming arrows to its start node are satisfied, not just the longest one.
Calculation Errors in LFT: Forgetting to subtract the duration from the LFT of the succeeding node is a frequent source of marks lost during the backward pass.
