Scheduling and Network Logic
Network Techniques
Understand the precedence diagramming method (PDM): activities as nodes, relationships as arrows. Know finish-to-start (FS), start-to-start (SS), finish-to-finish (FF), and start-to-finish (SF), and how lags modify each. Activity-on-arrow (ADM) uses dummy activities to preserve logic; know why dummies exist (unique activity identification and logic continuity).
Forward and Backward Pass
Master the arithmetic. Forward pass computes early start (ES) and early finish (EF): EF = ES + duration − 1 in day-count convention, or ES + duration in elapsed-time convention. Backward pass computes late finish (LF) and late start (LS): LS = LF − duration. Total float = LS − ES = LF − EF. Free float = ES(successor) − EF(activity) − lag. Critical path is the longest path, with total float equal to zero (or the minimum in the network).
Float and Near-Critical Paths
Negative float signals a constraint conflict; near-critical paths carry low float and are schedule risks. Multiple critical paths may exist; crashing one may reveal another.
Worked Example
A (3d) FS→ B (4d) FS→ C (2d). Forward: A ES 1 EF 3; B ES 4 EF 7; C ES 8 EF 9. Backward from finish 9: C LF 9 LS 8; B LF 7 LS 4; A LF 3 LS 1. Total float is zero throughout - all critical. Add a 2-day lag on A→B: B ES becomes 6, and float redistributes accordingly.
Traps
Watch for off-by-one errors, lags applied on the wrong relationship end, and confusion between total and free float.