August 1, 2010

Introduction to Program Evaluation and Review Technique (PERT Charts)

Program Evaluation and Review Technique
The Program (or Project) Evaluation and Review Technique, commonly abbreviated PERT, is a model for project management designed to analyze and represent the tasks involved in completing a given project.

Overview
PERT is a method to analyze the involved tasks in completing a given project, especially the time needed to complete each task, and identifying the minimum time needed to complete the total project.

PERT was developed primarily to simplify the planning and scheduling of large and complex projects. It was able to incorporate uncertainty by making it possible to schedule a project while not knowing precisely the details and durations of all the activities. It is more of an event-oriented technique rather than start- and completion-oriented, and is used more in projects where time, rather than cost, is the major factor. It is applied to very large-scale, one-time, complex, non-routine infrastructure and Research and Development projects.

This project model was the first of its kind, a revival for scientific management, founded by Frederick Taylor "Taylorism" and later refined by Henry Ford "Fordism". DuPont corporation's critical path method was invented at roughly the same time as PERT.

Conventions

  • A PERT chart is a tool that facilitates decision making; The first draft of a PERT chart will number its events sequentially in 10s (10, 20, 30, etc.) to allow the later insertion of additional events.

  • Two consecutive events in a PERT chart are linked by activities, which are conventionally represented as arrows in the diagram above.

  • The events are presented in a logical sequence and no activity can commence until its immediately preceding event is completed.

  • The planner decides which milestones should be PERT events and also decides their “proper” sequence.

  • A PERT chart may have multiple pages with many sub-tasks.

PERT is valuable to manage where multiple task are going simultaneously to reduce the redundancy

Terminology

  • A PERT event: is a point that marks the start or completion of one or more tasks. It consumes no time, and uses no resources. It marks the completion of one or more tasks, and is not “reached” until all of the activities leading to that event have been completed.

  • A predecessor event: an event (or events) that immediately precedes some other event without any other events intervening. It may be the consequence of more than one activity.

  • A successor event: an event (or events) that immediately follows some other event without any other events intervening. It may be the consequence of more than one activity.

  • A PERT activity: is the actual performance of a task. It consumes time, it requires resources (such as labour, materials, space, machinery), and it can be understood as representing the time, effort, and resources required to move from one event to another. A PERT activity cannot be completed until the event preceding it has occurred.

  • Optimistic time (O): the minimum possible time required to accomplish a task, assuming everything proceeds better than is normally expected

  • Pessimistic time (P): the maximum possible time required to accomplish a task, assuming everything goes wrong (but excluding major catastrophes).

  • Most likely time (M): the best estimate of the time required to accomplish a task, assuming everything proceeds as normal.

  • Expected time (TE): the best estimate of the time required to accomplish a task, assuming everything proceeds as normal (the implication being that the expected time is the average time the task would require if the task were repeated on a number of occasions over an extended period of time).
    TE = (O + 4M + P) ÷ 6

  • Float or Slack is the amount of time that a task in a project network can be delayed without causing a delay - Subsequent tasks – (free float) or Project Completion – (total float)

  • Critical Path: the longest possible continuous pathway taken from the initial event to the terminal event. It determines the total calendar time required for the project; and, therefore, any time delays along the critical path will delay the reaching of the terminal event by at least the same amount.

  • Critical Activity: An activity that has total float equal to zero. Activity with zero float does not mean it is on critical path.

  • Lead time: the time by which a predecessor event must be completed in order to allow sufficient time for the activities that must elapse before a specific PERT event is reached to be completed.

  • Lag time: the earliest time by which a successor event can follow a specific PERT event.

  • Slack: the slack of an event is a measure of the excess time and resources available in achieving this event. Positive slack(+) would indicate ahead of schedule; negative slack would indicate behind schedule; and zero slack would indicate on schedule.

  • Fast tracking: performing more critical activities in parallel

  • Crashing critical path: Shortening duration of critical activities


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Tomboy

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