FIT2002 Chap.4 WBS, Scheduling and Critical Paths
WBS, Scheduling and Critical Paths
Schedule planning begins by decomposing authorised scope, not by drawing dates. A deliverable-oriented Work Breakdown Structure reveals the complete work and creates work packages that can be estimated, assigned and controlled. Activities derived from those packages are connected by dependencies.
Network logic then shows possible paths, milestones and the sequence that currently controls the finish date.
PERT combines optimistic, most-likely and pessimistic estimates using (O + 4M + P) / 6. It does not remove uncertainty; it produces an expected duration from stated inputs.
The critical path is the longest-duration dependency path through the current network, while float describes scheduling flexibility outside that path. A strong solution lists complete paths, shows arithmetic and units, identifies the controlling path, and translates a changed duration into a management consequence.
What this chapter covers
- 01
Deliverable-oriented decomposition and the complete scope rule
- 02
Work packages as estimable and controllable units of work
- 03
Activities, dependencies, milestones and network sequencing
- 04
Three-point estimation using optimistic, likely and pessimistic values
- 05
Complete path totals and identification of the critical path
- 06
Float as scheduling flexibility rather than unused project time
- 07
Gantt schedules as time views built from validated network logic
Worked example · free
Calculate a PERT duration and detect a critical-path switch
- +2Apply PERT to B: (4 + 4 x 7 + 16) / 6 = 48 / 6 = 8 days.
- +2Total Path X as 3 + 8 + 5 = 16 days and Path Y as 3 + 10 + 4 = 17 days.
- +1Identify A-C-E as initially critical because 17 days is the longer complete path.
- +2After C rises to 13 days, recalculate Path Y as 3 + 13 + 4 = 20 days; Path X remains 16 days.
- +1Conclude that the forecast finish moves three days later unless approved action changes duration or logic.
Key terms
- Work Breakdown Structure
- A hierarchical, deliverable-oriented decomposition of the complete authorised project scope. The lowest planned elements become work packages suitable for estimating, assignment and control.
- Work Package
- A manageable component at the lower level of the WBS for which cost, duration, responsibility and completion can be estimated. It remains linked to a defined deliverable.
- Dependency
- A logical relationship that constrains the sequencing of activities. Dependencies may arise from technical order, resource choices, contracts or external conditions and should not be invented merely for convenience.
- PERT Estimate
- A weighted expected duration calculated as (optimistic + four times most likely + pessimistic) divided by six. Its usefulness depends on credible three-point inputs.
- Critical Path
- The longest-duration path through the current activity network. It establishes the earliest possible project completion under the stated logic and durations.
- Float
- The amount of scheduling flexibility available to an activity without violating a defined timing constraint. Float can change when durations or network relationships change.
WBS, Scheduling and Critical Paths FAQ
Why should the WBS be created before the schedule?
The schedule needs a trustworthy account of the work. A WBS decomposes authorised deliverables and exposes omissions before activities, durations and dates are assigned. Scheduling first can produce precise dates for an incomplete scope. After work packages are defined, the team can identify activities, dependencies, estimates, resources and milestones on a more defensible basis.
Is the critical path simply the sequence with the most activities?
No. Criticality depends on total duration and dependency logic, not the number of boxes. A path with three long activities can control the finish while a path with seven short activities has float. Write each complete path and sum its current durations. Recalculate whenever estimates, dependencies or scope change because the controlling route can switch.
What should I write after calculating a PERT value?
Show the formula, substitute all three estimates, calculate the expected duration with units, and use that value in the relevant path or schedule decision. Then interpret the result: identify any critical-path or milestone effect and state what must be monitored or approved. PERT is an estimating input, not proof that the activity will take exactly that long.
Assessment move
Start with one small scenario and carry it from deliverables to WBS, activities, dependency network and Gantt view. This prevents each artefact becoming an isolated diagram. Recalculate PERT examples by hand and label O, M and P before substitution. For every network, list complete paths in a table with their totals; circle the longest, then change one duration and do the entire comparison again.
Add units to each line and finish with a sentence about the forecast finish or available float. Check that milestones have zero duration and represent decision or completion points. When revising, deliberately create one incorrect network containing a missing activity or unjustified dependency and explain how the error distorts the schedule.
That diagnostic practice is valuable in scenario questions where the arithmetic may be simple but the planning logic is not.
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