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A Toolbook for Quality Improvement and Problem Solving (contents)

Gantt Chart: How to do it

The Quality Toolbook > Gantt Chart > How to do it

When to use it | How to understand it | Example | How to do it | Practical variations


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How to do it

  1. Define the requirements of the project or work to be planned. This should contain sufficient detail to enable good decisions to be made during the planning process.

    Defining precise requirements can be helped by identifying the constraints on the project, which typically fall into three areas:

    • Work, including tasks that must be completed and tasks that need not be completed. This may also include measures to determine the success of the project.
    • Resource, including people available, machines, tools, materials and general discretionary budget.
    • Time, including the calendar time by when the project must be completed, holidays and other periods when resources are not available.
  1. Identify all tasks that need to be completed to meet the requirements from step 1. A common way of doing this is to perform work breakdown using a Tree Diagram.
  2. When breaking the work down, aim to find clearly allocable chunks, where the required skills and resources are obvious. The duration of each task should be of comparable length and be short enough to enable clear tracking. For example, in a 12 month project, it is reasonable to have tasks of around 1 week each, whilst a 4 week project may be broken down into tasks which take no more than 1 day.

    Each task should be uniquely identifiable, either from its name or an allocated code. A simple scheme is to use a coding structure based on the work breakdown structure. For example, the drawing of a a motor housing in the design phase may be coded as DE.DR.MH01.

  3. For each task, identify what is required to complete it, such as specific skills and resources. This may also take into account what is known about the risks in the task. For example, it may be decided that a task with known or uncertain risks will require the involvement of a more skilled person (or even more people).
  4. Note that at this stage actual resources are not allocated; the information from this step is fixed, whilst actual resources may be shuffled around between tasks.

  5. Perform an initial allocation of resources to tasks, taking into account the requirements from step 3 and the resources actually available.
  6. A tool for matching tasks and people is the Resource Allocation Matrix.

    It may become clear at this time or later that there are insufficient resources to complete the project within the required time-frame, necessitating the acquisition of additional resources.

    Note that there is a chicken-and-egg situation between resources, estimation and allocation. For example, a skilled person may be initially estimated to take two days to complete a task, but then is found to be over-allocated in later calculations. As a result, a less skilled person is allocated, which changes the estimation to three days, which in turn changes the overall work profile of the project. This may result in several cycles of allocation, estimation and calculation before a satisfactory solution is found.


  7. Estimate the time that will be taken to complete each task, given the allocated resources. There are many ways of doing this, few of which are particularly reliable. The best method is to use data that has been collected from previous similar tasks (preferably by the same people).
  8. Identify task priorities and dependencies. This typically means finding those tasks which must be completed before each task can begin (although there are sometimes more complex dependencies).
  9. Independent tasks, which can be performed simultaneously, may be allocated priority values, for example to ensure that higher risk tasks are done earlier.

  10. Calculate the start and end dates for each task. For anything more than a simple situation, this will require the use of an Activity Network.
  11. A table, such as in the Fig. 1. may be used to organize the information from steps 2 to 7.


Fig. 1. Building information for the Gantt Chart


  1. Identify a scale for the Gantt Chart, for example one increment on the bar may represent anything from 1 hour to 1 month. Typically, this will enable the chart to be drawn within a single page width (although using the paper in landscape, rather than portrait format will give more width). For example, if there are 30 squares available for bars, and the project is calculated to take 6 months, then it may be decided that a single square will represent all or part of a week.


  3. Identify periods during which tasks may not be worked on, such as public holidays, private holidays, conferences and company events which must be attended. Mark these on the chart.


  5. Draw the horizontal bars on the Gantt Chart, against each task, using the information gained in steps 2 to 9.
  6. At a minimum, this will include an identification of each task (name or code) plus the bar for that task. It may also include columns for additional information, typically anything from steps 2 to 8, but most usefully the names of the people who will perform the task, as in the illustration.

    The bar for each task will fill each time column (identified in step 8), even if it represents only a part of this time. For example, in the illustration, the bar for 'Design new form' occupies two full day columns, even though it may only take a part of the second day. This potential confusion can be avoided by using the same scale for effort estimates and time columns, although this may result in a large number of columns or very coarse estimates. The best approach is usually to scale the time columns to fit on the page and be careful not to get confused by any task overlap.


Fig. 2. Building the Gantt Chart

  1. Investigate how well the available resources are used by summing down the time taken in each column into a Histogram for each resource, as in the illustration.
  2. This Histogram may then be used to perform load balancing by identifying actions to change tasks, resources, allocations and times that will enable the resources to be optimally used. For example, if a person is allocated to two tasks at one time, then either allocate one task to someone else or move it to a free period.

    After doing this load balancing, you may need to go back to step 5, repeating this process until a satisfactory chart is obtained.

  3. As the project progresses, regularly re-estimate outstanding work on tasks, taking note of the accuracy of previous estimates. Thus if most tasks so far took 20% longer than originally estimated, it is probably reasonable to add 20% to outstanding tasks. Other tracking activities may include adding new tasks, changing the resources available to the project, reallocating tasks (e.g. from people who are slow to people who are quick) and recalculation of the overall schedule.
  4. Progress on the project can be shown by publishing the completed chart, with a vertical line to show the current date, as in the illustration. All bars and part-bars to the left of the line are completed, and all to the right are yet to be done.



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