Schedule Planning and Development
From scope to a network
Development flows: WBS decomposition, activity definition, sequencing, duration and resource estimating, calendar assignment, then network calculation. Know the precedence diagramming method and its four relationship types (FS, SS, FF, SF), plus mandatory, discretionary, and external dependencies. Leads and lags modify relationships; a lead is a negative lag. Calendars matter: project, resource, and activity calendars can alter dates and even float.
The calculation
Perform a forward pass (early start, early finish) then a backward pass (late finish, late start). Total float = LS − ES (or LF − EF); free float = ES of the successor − EF of the activity. The critical path is the longest path through the network and normally has zero or minimum total float. Watch for near-critical paths, open-ended or dangling activities, hard constraints and deadlines, and negative float from imposed dates. Baseline only an approved, quality-checked model.
Worked example
A (5 days) precedes B (3) and C (4) finish-to-start; D (2) follows both B and C. Forward pass: A ES 0 / EF 5; B ES 5 / EF 8; C ES 5 / EF 9; D ES 9 / EF 11. Backward pass: D LF 11 LS 9; C LF 9 LS 5; B LF 9 LS 6; A LF 5 LS 0. Total float: A 0, B 1, C 0, D 0. Critical path A–C–D, duration 11 days.
Traps
- Confusing total float with free float.
- Calling the path with zero float "critical" when a constraint forced the float, not logic.
- Ignoring how multiple calendars can leave float on the longest path.
- Using lags to model work instead of activities.