Planning and Scheduling: Logic, Float, and Control
Network logic first
Planning defines scope, sequence, durations and resources; scheduling calculates dates. Decompose to a WBS, define activities with unique IDs and deliverable-based completion criteria, then sequence with precedence relationships (FS, SS, FF, SF) plus lags and leads. Activity-on-node (PDM) dominates modern practice; activity-on-arrow (ADM) requires dummy activities and is a classic source of path-counting and float errors.
Critical path and float
Run the forward pass for early dates and the backward pass for late dates. Total float = LS − ES (equivalently LF − EF); free float = earliest ES of successors − EF. The critical path is the longest path through the network and carries the minimum (usually zero) total float. Watch for multiple critical paths, negative float created by imposed constraints, and float considered to be owned by the path rather than by a single activity.
Resource and schedule control
Resource leveling shifts activities within their float and can change the critical path and finish date; resource smoothing uses available float without moving the project finish. Crashing adds cost for time and shows diminishing returns; fast tracking overlaps activities and raises risk. Supplement the deterministic CPM schedule with schedule risk analysis (PERT, Monte Carlo) and a documented schedule basis.
Traps
Assuming an activity with float is non-critical when it lies on another critical path; forgetting that a lag on an SS tie shifts the successor relative to the predecessor's start; reporting schedule status from a Gantt bar chart rather than the network.
Rule of thumb
Paths A–B–C = 5+3+4 = 12 days and A–D = 5+4 = 9 days: D has 3 days total float and 3 days free float, while B and C are critical.