PMGT5871 Chap.3 Activity Scheduling and the Critical Path
Activity Scheduling and the Critical Path
Scheduling turns the WBS into time through five steps: define activities, sequence them, estimate resources, estimate durations and develop the schedule. This chapter explains the four schedule formats and the node layout, then works a complete forward and backward pass to find early and late times, float and the critical path.
It closes with the Gantt chart, the difference between effort and duration, three estimating approaches and what scheduling software can and cannot be trusted to do. This is the most calculation-heavy topic in the first half of the unit and the one where small slips cost the most, because an error at one merge node carries through every later activity.
The Week 3 tutorial asks groups to choose a planning level for the first case study, build a node for each activity, draw the precedence diagram and complete a resource table, so the same skills feed the group plan.
What this chapter covers
- 01
Schedule tables, milestone charts, network diagrams and Gantt charts
- 02
Dependencies, milestones, float and the critical path
- 03
The five-step scheduling process
- 04
Merge and burst activities
- 05
Forward pass and backward pass
- 06
Effort compared with duration
- 07
Expert judgement, parametric and AI estimating
Worked example · free
Calculating float and the critical path
- 1Forward pass: A 0 to 4, B 4 to 10, C 4 to 9.
- 1Merge rule: D takes EF 10 and ends at 13, E ends at 11, F starts at 13 and ends at 15.
- 1Backward pass: F 13 to 15, D 10 to 13, E 11 to 13, B 4 to 10.
- 1Burst rule: C takes the lowest successor LS, 10, so its LS is 5; A has LS 0.
- 1Float = LS minus ES: C 1, E 2, others 0; critical path A, B, D, F.
Key terms
- Early start
- The earliest time an activity can begin, found in the forward pass from its predecessors.
- Late finish
- The latest time an activity can end without delaying the project, found in the backward pass.
- Merge activity
- An activity with two or more predecessors, which takes the highest predecessor early finish.
- Burst activity
- An activity with two or more successors, which takes the lowest successor late start.
- Duration
- The total elapsed time to complete an activity, including lead and lag times.
Activity Scheduling and the Critical Path FAQ
How is float calculated?
Float is the late start minus the early start, which equals the late finish minus the early finish. It is the time an activity can slip without moving the project end date, and activities with zero float form the critical path.
What is the difference between effort and duration?
Effort measures the hours people actually spend on the work, and it drives cost. Duration is the total elapsed time to finish the activity, including lead and lag times, so one day of effort can take several days of duration.
Should duration estimates include contingency?
No. The Week 3 slides say estimates should reflect schedule risk realistically, should not include contingency time, should not be overly optimistic and should be revised as the project progresses. Contingency belongs in the cost and risk plans where it stays visible.
What is the critical path?
It is the chain of critical activities through the network, meaning activities with zero float. Any delay to one of them delays the project completion date, so the critical path also gives the shortest possible project duration with the current logic.
What do scheduling tools such as MS Project automate?
They use the entered tasks, durations and dependencies to draw the network diagram and Gantt chart and to calculate the forward and backward passes and the critical path. The output is only reliable if scope, dependencies, resources and durations are complete and realistic.
What happens if a non-critical activity slips by more than its float?
It becomes critical and delays the project by the excess. In the pump example, the cable run has two days of float, so a three-day delay to it pushes commissioning back one day and the project finishes on day 16 instead of day 15.
Exam move
Draw small networks of five to seven activities by hand and run both passes until you never hesitate at a merge or a burst node. After each attempt apply the three checks: zero float on the start and finish, critical durations adding up to the project length, and float equal on both formulas. Then convert one network into a Gantt chart with float shown as a tail, and change one duration to see whether the critical path moves.
For the qualitative questions, make a two-column card for effort and duration and another for the three estimating approaches with the conditions each one needs. When a question gives a table instead of a diagram, sketch the network first anyway; it takes a minute and prevents most sequencing mistakes.
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