Planning & Decision Making · Topic 5 of 11
Gantt Charts
Theory
While a PERT chart is excellent for calculating the maths behind a project, a Gantt Chart is a much better visual communication tool. It looks like a bar chart and makes it very easy to track day-to-day progress and schedule staff or resources.
1. The Layout of a Gantt Chart
- Y-axis: The tasks are listed vertically down the side (usually from A to Z).
- X-axis: The time scale (hours, days, weeks) runs along the bottom.
- Task Blocks: A solid rectangular block is drawn to represent the exact duration of an activity. The block begins at the activity's Earliest Start Time (EST).
2. Representing Float Time
If an activity is essential (not critical), it will have float time. This means there is a gap between when the activity finishes and its Latest End Time (LET).
On a Gantt chart, float time is usually represented by a lightly shaded box or a thin line extending out from the end of the solid duration block, stretching all the way to the LET.
Massive Exam Trap:
The marking instructions state that colours cannot be used in exams. If you are asked to draw float time, you must use a different pattern (like cross-hatching) or a thin line, rather than switching from a blue pen to a red pen! Sometimes, an exam question will explicitly ask you to draw a Gantt chart without float times, so always read the question carefully.
3. Gantt vs PERT
You must be able to state the advantages and disadvantages of both charts:
- Gantt Advantages: Highly visual, task durations can be easily compared at a glance, and it is very easy to see how many tasks are happening at the exact same time (great for scheduling workers).
- Gantt Disadvantages: Dependencies (which tasks rely on others) are much harder to visualise compared to the arrows on a PERT chart.
Worked examples
Example 1
Example 1: Interpreting a Gantt Chart
A project manager creates a Gantt chart for a garden landscaping project. The time is measured in days.
- The solid block for Task C starts at day 4 and ends at day 9.
- A thin line (representing float) extends from the end of Task C at day 9, finishing at day 12.
(a) State the Earliest Start Time and the Duration of Task C.
(b) State the float time of Task C.
(c) State the Latest End Time of Task C.
(a) The Earliest Start Time is 4 days. The duration is 5 days (from day 4 to day 9).
(b) The float time is 3 days (from day 9 to day 12).
(c) The Latest End Time is 12 days.
Example 2
Example 2: Constructing a Gantt Chart from ESTs
A student is planning to build a custom PC. They have calculated the following Earliest Start Times (EST) and durations (in hours):
- Task A: EST = 0, Duration = 2
- Task B: EST = 2, Duration = 3
- Task C: EST = 2, Duration = 1
- Task D: EST = 5, Duration = 2
Describe exactly how to draw the solid blocks for Tasks A, B, C, and D on a Gantt chart.
- Task A: Draw a solid block starting at 0 and finishing at 2.
- Task B: Draw a solid block starting at 2 and finishing at 5.
- Task C: Draw a solid block starting at 2 and finishing at 3.
- Task D: Draw a solid block starting at 5 and finishing at 7.
Example 3
Example 3: Scheduling Resources (Exam Style)
A bakery is preparing a massive catering order. A Gantt chart is created to schedule the tasks. Looking at the chart, the manager notices that on Day 4, the solid blocks for Task B, Task D, and Task E overlap simultaneously. Every task in the bakery requires exactly one person to complete it, and a person can only work on one task at a time.
(a) State the minimum number of staff required to work on Day 4 to keep the project on schedule.
(b) Task E is an essential activity with a float time of 2 days. The manager only has 2 staff members available on Day 4. Explain how the manager could alter the schedule so the project is not delayed.
(a)
Because three tasks overlap on Day 4, the bakery needs a minimum of 3 staff members.
(b)
Because Task E has a float time of 2 days, the manager can simply delay the start of Task E until Day 5 or Day 6. This reduces the number of simultaneous tasks on Day 4 down to two, meaning the 2 available staff members can handle it without delaying the overall project.