Gantt Charts
What are Gantt Charts?
- Gantt charts show you in an instant if a project is on schedule. Consider building a house. A Project Manager would start by writing down each of the stages and then estimating how many days it would take them to complete that stage.
- If you did one job after another in sequence, you could add up how long each job would take to build the house - 30 days. On the other hand, if all the tradesmen could work at the same time, then to build the house would take only as long as the longest job - 5 days. In fact, some jobs have to finish before others begin whilst others can be done at the same time. For example,
- You cannot do the foundations until the ground has been prepared.
- You cannot build the walls until the foundations are complete.
- You cannot decorate until the walls have been plastered.
- The roof, plumbing, electrics, doors and windows can all be done at the same time, as soon as the walls are built.
- You can start the garden and drive (arguably) as soon as the foundations have been laid and at the same time the walls are being built.
- So how do you estimate how long the house will take to build? You produce a Gantt chart.
Problems with Gantt Charts
- It can be difficult to estimate how long jobs will take and drawing a Gantt chart assumes that the Project Manager can estimate the time each task takes reasonably accurately.
- Estimating how long jobs take improves with experience but it is always difficult!
- The most important drawback with Gantt charts, however, is that they don't tell you which jobs MUST get finished on time, or the whole project will fall behind.
- Some jobs are critical and must get done on time. For example, in the house-building example, getting the walls built might be deemed critical because so many other jobs depend on that one getting done on time. If the garden and drive take a little longer than planned, then the project can still be finished on time - it isn’t a critical task.
- Gantt charts do not highlight which jobs are critical. Another method is needed for this. It is known as Critical Path Analysis, or CPA.