Project Planning and Scheduling
Network logic and float
Planning defines what must be done; scheduling places it in time. Build a WBS, define activities, establish dependencies, assign durations and calendars, then calculate the network. In critical path method (CPM), the forward pass gives earliest start and finish (ES, EF); the backward pass gives latest start and finish (LS, LF). Total float is LS − ES or LF − EF. Free float is the time an activity can slip without delaying its successor.
The critical path is the longest path through the network and sets project duration. Activities on it normally have zero total float. Traps include assuming the path with the most activities is critical, confusing total float with free float, and treating lags as activities. Calendars, resource leveling, and constraints can change float and even create a new critical path.
Worked example
Consider two paths: A–B–C with durations 5, 5, and 5 days (15 days total), and D–E with durations 8 and 6 days (14 days total). The critical path is A–B–C at 15 days. Path D–E has 1 day of total float. If D–E slips 2 days, it becomes 16 days and the critical path shifts. Rule of thumb: project duration equals the longest path, not the sum of all activity durations.