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Cost and Optimization

Activities are not allowed to share the same starting and ending nodes, so sometimes a “dummy activity,” which has no cost or time duration is inserted to make the precedence of tasks clear. Activities that happen concurrently are laid out in parallel on the diagram.

Harold Schroeder, a Consultant with expertise in managing complex projects, said a common mistake in PERT diagrams is forgetting to connect all activities with another. “This is often called ‘dangling’ and can be easily avoided by checking to ensure all on-final activities are connected to some other activity in the network. That said, PERT allows one to see how the pieces fit together, thereby revealing critical elements that may need correction,” he noted.

  1. Calculate how long the project will take. Add up the likely durations of all the tasks to get the expected length of the project. You can then identify start and end dates. Working with your optimistic and pessimistic time estimates, you can also forecast the soonest possible and latest possible completion dates. If it is imperative your project finish by a certain date, choose that as the completion date and work backwards using the most pessimistic time estimates.
  2. Figure out the critical path and slack. The critical path is the sequence that takes the longest to complete. Slack is the amount of time an activity can be be delayed without delaying the overall project (it is calculated as the difference between the earliest possible start time and the latest possible start time or between the earliest finish time and the latest finish time). Any activity with a slack time of zero is on the critical path. Some PERT charts color the critical path in red.