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The Gantt Chart

Quality Tools > Tools of the Trade > 13: The Gantt Chart


The Gantt Chart, named after its originator, Henry L. Gantt, is a simple and widely used method of working out and displaying the knotty problem of who does what and when. Typically it is used in a project where a number of people are working on interlinked tasks.


How does it work?

The basic principle of the Gantt Chart is to show time in a number of columns and to draw bars across those columns in order to show the usage of time. Each bar represents a task being carried out by one or more people. The bars can be summed downwards into simple histograms to show whether individuals are double-booked or have spare time on their hands.



Gantt chart and histogram

Additional symbols and colours may be added to the chart to show events and features such as milestones (zero-length tasks), critical paths (which, if extended, will slip the completion date of the project) and float (where tasks can be slipped without jeopardizing the completion date).


How do you do it?

1. Break down the task into units that can be clearly allocated and tracked. In a typical project this might be tasks of one week or less, allocated to one person each (although this can vary widely with the type of project).


2. Identify skills and resources required by each task and allocate the task to a person or group, along with appropriate resources. Be careful when allocating scarce resources, such as subject specialists or expensive equipment; a person with less skill who can perform the task, albeit over a longer period, can free up the expert for working on more critical activities.


3. Revise the time estimate for each task in the light of the skills of the people on the task. Beware of optimistic estimation. It can be helpful later if a 'safety factor' of anything from 10% to 100% is added to allow for slippages. It is usually more acceptable to complete a project ahead of schedule than have to continually move out the completion date.


4. Identify dependencies between tasks and other prioritisation that will allow you to put the tasks into the correct order. For example, it is better to do high-risk tasks as early as possible, to give you time to react to unexpected difficulties.


5. Draw up a blank Gantt Chart with enough lines on for all identified tasks and with columns spanning the range of times and/or dates to be displayed. Make the period between each column in the same range as the average task length, for example if most tasks are from one to five weeks, make each column one week (provided there is space on the paper).


6. Write the tasks into the chart and add bars to indicate the start time and duration of each tasks. This is where the real skill in scheduling is found as you need to position tasks carefully to ensure that dependencies (where one task must follow another task) are preserved and that people and other resources are optimally used.


The Gantt Chart can be further enhanced to show the hierarchy of work breakdown, other project data and dependencies between tasks, as below.



Enhancing the Gantt chart

Gantt Charts can be drawn automatically with project management software such as Microsoft Project. Spreadsheets or wordprocessor tables can also be used  to generate the chart in table format.


Next time: Post-it® Notes and the Post-Up


This article first appeared in Quality World, the journal of the Institute for Quality Assurance

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