Gantt, PERT, Critical Path

Scheduling a project

Think of making Sunday lunch. The roast takes two hours; the vegetables take thirty minutes; the gravy needs the roast’s juices; you can’t carve until the roast is done. Some jobs happen at the same time, some must wait for others, and one long chain of jobs decides when lunch is on the table.

Project schedules work the same way. Three tools help.

Gantt charts

A Gantt chart shows each task as a bar along a timeline, often with milestones and percentage complete. It’s easy to read and good for tracking progress against the plan.

Its weakness: dependencies are hard to see. You can’t tell from a Gantt chart alone which delays will push out the end date.

PERT: estimating uncertain tasks

The Program Evaluation and Review Technique models the project as a network of tasks, and uses three estimates for each task’s duration:

Expected time = (Optimistic + 4 × Most likely + Pessimistic) / 6

Optimistic 4 days, most likely 6, pessimistic 14; expected time 7 days.

The most likely guess counts four times; the best and worst cases count once each. That allows for the range without letting one extreme dominate.

The critical path

The critical path is the longest chain of dependent tasks through the project. Its length is the shortest time in which the whole project can be finished.

A small network: path A-B-D takes 18 days and is critical; path A-C-D takes 14 days, so C has 4 days of slack.

Tasks on the critical path have zero slack: delay any of them by a day, and the project finishes a day late.
Tasks off the critical path have slack (also called float): they can slip by that much without delaying the project.
The critical path can change as tasks finish early or late, so it’s recalculated as the project goes.

In the example, the critical path is A-B-D (18 days). Task C has 4 days of slack. Putting more people on C won’t finish the project any sooner; speeding up B will.

Speeding things up

Crashing: adding people or money to critical-path tasks to shorten them. It costs more.
Fast-tracking: running tasks in parallel that were planned one after another. It adds risk.

Timebox management

Timeboxing fixes the delivery date and budget, and adjusts scope to fit, delivering the most important features first. It’s used in agile and RAD. It prevents schedule overruns, but needs disciplined prioritisation.

How the exam asks about it

If the question asks…	The answer is usually…
What the critical path represents	The longest path, setting the minimum project duration
Slack on critical-path tasks	Zero
Technique using three time estimates	PERT
Weakness of a Gantt chart	It doesn’t clearly show dependencies or the critical path
Technique that fixes time and varies scope	Timebox management

In short

A Gantt chart shows tasks as bars on a timeline; good for tracking progress, weak at showing which tasks depend on each other.
PERT uses three estimates per task (optimistic, most likely, pessimistic): expected time = (O + 4M + P) / 6.
The critical path is the longest chain of dependent tasks; its length is the shortest possible project duration.
Tasks on the critical path have zero slack: any delay delays the project.
Tasks off the critical path have slack (float): they can slip a bit without delaying the project.
Timeboxing fixes the deadline and adjusts scope to fit, as in agile and RAD.

Exam traps

The critical path is the LONGEST path, not the shortest.
Speeding up a task that isn't on the critical path doesn't finish the project sooner.
PERT handles uncertainty with three estimates; CPM uses one estimate per task.
A Gantt chart alone doesn't show which delays matter.

Slides

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