PMGT5887 Chap.7 Task Dependencies, Leads, Lags and the Critical Path
Task Dependencies, Leads, Lags and the Critical Path
Scheduling questions are the most calculation-heavy part of the unit, and this chapter gives them a full treatment. It starts with the precedence diagramming method behind every Gantt chart and the four dependency types, finish-to-start, start-to-start, finish-to-finish and start-to-finish, each with an example of our own.
It explains leads and lags and how Microsoft Project enters them in the Predecessors column, including percentage leads and offset starts, and how summary tasks, recurring tasks, milestones, constraints and the timeline view shape a schedule for stakeholders.
The core is critical path analysis: a nine-task network solved by forward and backward pass, with every earliest and latest start and finish printed, two tied critical paths, slack on the one task that has it, path listing as a cross-check, and two what-if delays. It ends with how Microsoft Project highlights critical tasks, adds a Total Slack column and switches between Gantt and network views.
Every number in the network can be recomputed by hand from the chapter.
What this chapter covers
- 01
The precedence diagramming method
- 02
Four dependency types
- 03
Leads, lags and the Predecessors column
- 04
Summary, recurring and milestone tasks
- 05
Critical path analysis
- 06
The forward and backward pass
- 07
Slack and what-if delays
- 08
Seeing the critical path in Microsoft Project
Worked example · free
Applying a lag and finding the new finish
- +1Pour slab runs from day 0 to day 3.
- +1With a 4-day lag, Erect frame can start no earlier than day 3 + 4 = 7 from the slab side.
- +1Order windows runs from day 0 to day 6, so it does not delay the frame; Erect frame starts at day 7 and finishes at day 12.
- +1Order windows could finish as late as day 7, so its slack is 7 - 6 = 1 day.
Key terms
- Finish-to-Start
- The default dependency, in which the successor cannot start until the predecessor finishes.
- Lag
- Waiting time inserted between linked tasks, delaying the successor.
- Lead
- An overlap that lets a successor start before its predecessor is complete, entered as a negative lag.
- Forward Pass
- The calculation of earliest start and finish for every task, from the project start onward.
- Backward Pass
- The calculation of latest finish and start for every task, from the project end backward.
- Milestone
- A task with zero duration that marks a significant point, such as a signed contract.
Task Dependencies, Leads, Lags and the Critical Path FAQ
Which dependency type does the Link Tasks button create?
Finish-to-start: the next task waits for the previous one to end, and it is the relationship the homework creates by selecting tasks and clicking Link Tasks. The other three types describe overlapping or hand-over relationships.
How do I enter a lead in Microsoft Project?
As a negative lag in the Predecessors column, for example 3FS-2d for two days of overlap, or as a percentage such as 2FS-50% to start halfway through the predecessor.
Can a project have more than one critical path?
Yes. When two chains through the network share the same longest duration, both are critical, every task on them has zero slack, and a delay on either chain delays the project finish.
What does a task with zero slack mean?
Any delay to that task delays the whole project, because it lies on the critical path. Tasks with positive slack can slip by that amount without moving the finish.
What is a recurring task in Microsoft Project?
A task that repeats on a pattern for part or all of the project, such as a daily stand-up or a two-hour progress report every Monday. It is entered once and Project creates every occurrence.
Assessment move
Draw small networks of six to nine tasks and run both passes by hand every few days, timing yourself. Check each answer by listing paths and by confirming that the end task's latest and earliest finish match. Practise reading Predecessors entries such as 4SS+1d aloud. In Microsoft Project, rebuild one of your hand networks and compare the Total Slack column with your own numbers.
Keep a sheet of the test rules: take the larger value going forward, the smaller going back, calculate slack as latest start minus earliest start, and read a lead as a negative lag. Review it before the test so you do not reverse a merge under time pressure.
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