The GANTT diagram is a tool that allows the user to model the planning of tasks necessary to the performance of a project. This tool was invented in 1917 by Henry L. Gantt.
Given the relative ease of reading of GANTT diagrams, this tool is used by almost all project managers in all sectors. The GANTT diagram is a tool for the project manager, which allows for the graphic representation of the progress of the project, but it is also a good means of communication amongst the various persons involved in a project.
This type of modeling is particularly ease to implement with a simple spreadsheet, but there are also specialized tools, the best-known being Microsoft Project. There are also free equivalents of this type of software.
In a GANTT diagram, each task is represented by a line, while the columns represent the days, weeks, or months of the schedule depending on the duration of the project. The estimated time for a task is modeled by a horizontal bar, the left end of which is positioned on the intended start date and the right end of which is on the intended end date. Tasks can be placed in sequential chains or carried out simultaneously.
If tasks are sequential, priorities can be modeled using an arrow from the upstream task to the downstream task. The downstream task cannot be carried out until the upstream task has been completed.
As a task progresses, the bar that represents it is filled proportionallz to its degree of completion. Thus, it is possible to get a quick overview of the progress of the project bz tracing a vertical line across the tasks at the level of the current date. The completed tasks are located to the left of this line; tasks that have not yet begun are at the right, while the tasks being carried out are crossed by the line. If there filling is located to the left of the line,the task is in delay compared to the plan!
Ideally, such a diagram should not have more than 15 or 20 tasks so that it can fit on a single A4 page. If the number of tasks is greater, it is possible to create additional diagrams detailing the planning of the main tasks.
Additionally, it is possible to have the major events other than the tasks themselves show on the plan as points of connection for the project: these are the milestones.
The milestones allow the project to be decided in clearly identifiable phases, which avoids the end of the project being too far off (this is often referred to as the "tunnel effect", a project with a long duration and no intermediate endpoint). A milestone could be the production of a document, the holding of a meeting, or a deliverable of the project. The milestones are tasks of zero duration, represented on the diagram by a specific symbol, most frequently a reverse triangle or a diamond.
It is generally possible (and useful) to show resources, human or material, on the diagram, in order to allow for estimation of needs and give an idea of global cost.