If you've ever had to manage or work on a multistep project, you may have encountered a Gantt chart or a program evaluation and review technique, or PERT, chart. These charts are tools that help you visualize the activities involved in completing a project. They combine scheduling information with dependencies among tasks, but they do so in different formats. Gantt charts present tasks in sequential order, with start and end dates. PERT charts are flow charts that are generally more complex and best suited for larger projects.
Gantt charts are bar graphs. The X-axis contains dates and the Y-axis lists separate tasks. On each line of the Y-axis, the chart depicts a bar positioned to extend from the task's start date to its end date. Tasks are listed in start date order. PERT charts are network diagrams that use boxes to represent tasks and arrows to present dependencies between tasks. The boxes are laid out from left to right, but there is no fixed Y-axis with dates. The first box, or root, is centered vertically on the left side, and the subsequent tasks can be drawn anywhere along the Y-axis. Arrows can point to the right, up or down, but never to the left.
A dependent task is one that cannot start until another task is partially or completely finished. A Gantt chart can list subtasks grouped by task, which implies a sequence of dependencies. The sequence can be made explicit by drawing an arrow from one task to a dependent task. PERT chart dependencies always require arrows. A task box can point to multiple dependent tasks. Furthermore, dependent tasks may have multiple incoming arrows when several tasks must finish before the dependent task can start.
The Y-axis of a Gantt chart functions as a calendar, with equal-length segments representing units of time, such as days, weeks or months. This arrangement helps managers know when to start tasks and to quickly recognize when tasks are not on schedule. The spacing between task boxes on a PERT chart doesn't have to be proportional to the start and end dates, making the chart less convenient for managing deadlines. Often the arrows are labeled with time units. In computerized PERT charts, you click on a box to get the task details, including projected start and end dates.
PERT charts can operate at multiple nesting levels: A top-level diagram shows the major tasks and lower-level diagrams show the subtasks associated with a task. This provides an easy method to add or remove subtasks without disturbing the top-level diagram. Gantt charts normally show all tasks and subtasks at the same level, which can create multipage charts that managers must rearrange whenever adding, rescheduling or removing tasks. The multipage arrangement makes it difficult to draw dependency arrows, which usually limits the use of Gantt charts to projects with no more than 30 activities.