Most project management software can produce a PERT (Program Evaluation and Review Technique) chart but what is it and how can a project manager use it to best effect?
A PERT chart is simply a visual representation of a project’s schedule. It is a technique rather than a tool so, in theory could be done with just pencil and paper, although in practice this is unlikely. It displays all of the project tasks and shows which ones need to be done sequentially because of inter-dependencies and which tasks can be performed in parallel to minimise the total time taken to complete the project. Importantly it also shows the critical path of activities which need to be finished on schedule for the project to stay on track.
You can create a chart with a range of attributes assigned to tasks, for example, each task can have an estimate of the least time taken to finish and the most time taken to finish. This is one of the reasons that make a PERT chart more effective than Gantt charts in that they can show a more realistic schedule of events and help the project manager manage the expectations of clients and stakeholders better. It can also help identify where potential problems could arise if one task is likely to prevent other dependent tasks from starting.
To make the best use of a PERT chart you need to identify all the tasks or activities necessary to complete the project. In the initial planning phase it is important to ensure that people with the right technical knowledge are involved so that not only are estimates realistic but to ensure that no tasks are missed off the initial schedule.
Start by getting everyone together and sequentially identify each task; at this stage it may not be necessary to break down complex tasks or tasks requiring more than one person to complete, providing the estimate is realistic but note which tasks need more detail and sub-tasks and come back to them to add detail later.
Once all the tasks are identified (don’t necessarily expect to do this in one session) then you can start to look at which ones can be done in parallel to save time if you have enough people to do this. Remember that dependencies can be the result of not having adequate staff to complete tasks simultaneously; they don’t have to be technical in nature.
Once you have worked through all the tasks then you need to obtain estimates for each task. Unless you have completed very similar tasks in the past these estimates will be based on the experience of the people involved and, as they are estimates, you can expect them to change, sometimes drastically, once the project is underway.
When each task has an estimate you can begin to form a schedule and add attributes such as start and end dates, and begin to determine the project’s critical path. The critical path shows which tasks must start and end on schedule in order to prevent significant delays in the overall project completion. It is monitoring and controlling these critical tasks that should be the main focus of the project manager.
As with any technique and any tool, a PERT chart is not perfect but can be used in conjunction with other techniques to help you manage a project more effectively.
Michelle
Precedence diagrams were developed in 1956/57 Kelly and Walker started developing the algorithms that became the ‘Activity-on-Arrow’ scheduling methodology for DuPont. The program they developed was used in trials on plant shutdowns in 1957 and their first paper on Critical Path Scheduling (CPS) was published in March 1959. PERT seems to have been developed at the same time to look at the uncertainties involved in the critical path of a project by using three point estimates for the durations.
No I think Microsoft was just confused about the correct terms to use when they developed Microsoft Project.
Precedence Diagrams were developed after PERT charts and are more accurate at showing the relationship between different tasks so maybe that’s why Microsoft uses them rather than PERT charts, which assume that one task cannot start until the previous one is finished, who knows…
It would be worth clarifying exactly what the various planning techniques are and highlighting their differences, advantages and disadvantages for particular uses.
Michelle It is interesting that what Microsoft call a PERT chart is not infact a PERT chart at all. PERT is a statistical way of calculating expected project end date based on three point estimates of project duration. It was developed in the 1950’s for the U.S. Navy’s Polaris nuclear submarine project.
What Microsoft project call the PERT chart is actually a precedence diagram showing the dependencies between tasks.