Gantt Chart tutorial

Also see PERT practice

Also see real Gantt/PERT questions and solutions in these previous exams... 2001, 2002, 2003

"Hopeful Secondary College School Council decided at its last meeting that a new computer laboratory was required. The project commenced on Monday 16 October. The plan for the process and its progress are shown in the diagram above. The school council has decided to manage to refurbishment of the room and is required to have the room completed by Friday 22 December. Due to class requirements, the commencement of alterations to the classroom cannot commence before Monday 11 December."

First: this Gantt chart was produced using Microsoft Project. This is a common tool for producing Gantt charts, but it is only one way of doing so: other charts may look a bit different.

Look at the time scale at the top of the chart. Notice anything odd? Yes, folks. The minor date scale (under the line beginning "16 Oct '00") shows every fourth day. Makes it much harder to read. Heaven knows why the examiners did it (I can guess: it was the only way they could get it to fit onto the printed page.)

Anyway, just remember to always check the time scale before interpreting a chart.

The major scale is in units of fortnights (the first step is 16 Oct, the next is the 30th - 14 days apart.) The dotty lines going down the chart mark beginnings of major time units. e.g. task 8 begins on the dotty line that marks the beginning of major unit beginning December 11 so you can easily tell it starts on Dec 11.

There are 13 tasks, listed downwards in the left column.

The blue bars graphically show the length of each task (which is also visible numerically in column 2)

The arrows from the end of one task to the beginning of another mean the second task must wait for the first task to end before the second can begin. This is called a "dependency". The earlier task is called the "predecessor".

Most tasks follow each other in a "lock-step" manner until you reach tasks 10 and 11 which overlap: overlapping bars mean the tasks can be carried out concurrently (at the same time.)

Both tasks 10 and 11 are dependent on task 9. Task 11 starts on the same day as task 10 and lasts one workday longer.

Notice however that task 10's bar is a fair bit shorter than task 11's even though they are only one workday different: this is because task 11 spans a weekend, so its bar is actually 5 days long even though no work is done on the weekend.

The diamonds show milestones. Milestones are significant points of progress in the project. They have zero length: no work is actually needed for a milestone. They mark major points of project development.

The black lines in the bars of the first 3 tasks indicate how much of the task has been completed so far. You can see tasks 1 and 2 are finished and task 3 is half-finished. This lets a project manager keep track of where the project is actually up to.

Quick question 1: When does the project start?

Answer: 16 October (see where task 1's bar begins on the dotty line that starts off "16 Oct")

Quick question 2: When does the project end?

Answer: 21 December (the milestone is dated so you can easily tell) [actually, I believe the milestone is wrong - see below]

Quick question 3: The exam questions below are worth a total of 5 marks: how long should you spend on them?

Answer: For 2003 and beyond, the exam is worth 90 marks and you have 120 minutes, therefore 1.333 minutes per mark.

Here's the chart again so you don't have to scroll so far...

Exam Question 1: What is the first milestone in this project? (1 mark)

Exam Question 2: How much 'slack' time is there after commencement of the alterations, including weekends? (1)

First let's find when the alterations commence: this is task 8, "Prepare site" on 11/12/00.

Now let's mentally expand the time scale (you could mark the days off with a pen). Note that while task 11's bar spans the weekend and is 5 days long in total, only 3 of the days are work days.

Let's find the critical path, starting at task 8: what is the longest possible path of tasks until the end? Tasks 8,9,11,12 add up to 1+2+3+1=7 days.

The alternate path (since tasks 10 and 11 overlap) is Tasks 8,9,10,12 adding up to 1+2+2+1=6 days.

So, there is a 1 day difference between the paths BUT the question adds "including weekends." So, if task 10 really got in trouble, and they needed all the time they could muster without affecting the finish date, they could work the weekend as well, giving them 3 slack days maximum.

Another way to approach it is to just looking at where the 2 tasks diverge (14 Dec) until the point they converge to a single path again at task 12 on 19 Dec (the same technique used in the PERT chart tutorial) The shorter path [task 10] is 2 days and the longer path [task 11] is 3 days - a one day difference. Add on the weekend as potential working time, as the question says, and the slack is 3 days.

(If you look at the original printed exam, it is VERY hard to read any detail from it and teachers have given the examiners detention for this. If there is a Gantt chart in the next exam, it must be easier to read than the one in 2000: it couldn't be worse!)

Exam Question 3: The supplier of the cable has just informed you that the cable delivered to you is faulty and the replacement cable will not arrive until the end of Monday 18 December. What impact will this have on the end date? (1)

THIS DISCUSSION IS SUBJECT TO REVIEW ONCE I WORK OUT WHERE THE BIG APPARENT PROBLEM IS!

The examiners' answer to this question was a mess. They ended up accepting 3 different answers, because many students interpreted "end date" differently.

Their official answer was: "'[The end date would be delayed by] 1 day - cabling will now start on Tuesday 19th not Monday 18th but project is still able to be completed on time' or 'nothing' or 'end date will be delayed by one day' or 'end date will still be met'."

Now call me a rebel, but I think this answer is a pile of fetid dingo's kidneys. How can it make sense?

Here's the end of the Gantt chart in more detail:

Let's take it step by step...

1. According to my calculations, the Gantt chart's final milestone is wrong. It is marked 21 Dec, but it should be on the 20th.

2. Since the cabling should have started on the 14th of December (not the 18th as the examiners' answer claims), a delivery delay causing installation to start on December 19th would make the end of the Gantt chart look like this:

Therefore, the finishing date of the project would have changed from end end of Tuesday 19 Dec to the end of Friday 22 Dec - a total of THREE days' delay to the 'end date', if you interpret 'end date' as the original planned finishing day: December 19.

If you interpreted 'end date' as the date the room had to be finished by (Dec 22 in the case study), there is no change to the 'end date'.

This fuzzy definition of 'end date' was the reason the examiners ended up with a bag of acceptable answers to the question.

Anyway, the examiners accepted 3 different answers (not including mine above.) I still think mine is valid, if interpreting 'end date' as 'original expected finishing date' (Dec 19).

I would be VERY grateful if any smart bunnies out there could alert me to faults in my theory about this question!

Exam Question 4: What options would a project manager have if the cable was arriving on 21 December and the due date still had to be met? (2)

This answer has nothing to do with the Gantt chart: it is a common question to test common sense. It is really asking, "How could a task be hurried up?" Expect this type of question and remember that it is not related to the Gantt chart you are given for the other questions.

The sorts of things you can do are:

• work 24 hours a day;
• bring in more workers;
• reduce the time taken on other tasks (such as cable testing);
• improve work efficiency (e.g. use an electric screwdriver to tighten screws rather than using a manual screwdriver or replace inexperienced workers with skilled and experienced staff who would work faster.)

Methods that 'cut corners' and affect the quality of the work are not recommended! Suggestion 3 above (cutting down on testing time) is perilously close to 'cutting corners.'