The Program (or Project)
Evaluation and Review Technique
PERT
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 of PERT
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.
This model was invented by Booz Allen Hamilton, Inc. under contract to
the United States Department of Defense's US Navy Special Projects
Office in 1958 as part of the Polaris mobile submarine-launched
ballistic missile project. This project was a direct response to the
Sputnik crisis. Some US government contracts required that PERT be used
as part of management supervision.
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 R&D-type projects where time, rather than cost, is the
major factor.
This project model was the first of its kind, a revival for scientific
management, founded in Fordism and Taylorism. Only DuPont corporation's
critical path method was invented at roughly the same time as PERT.
The most recognizable feature of PERT is the "PERT Networks", a chart of
interconnecting timelines. PERT is intended for very large-scale,
one-time, complex, non-routine projects.
PERT terminology and conventions
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.
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
* 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 (rhymes with "feed", not "fed"): 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
* 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)
References
http://en.wikipedia.org/wiki/PERT
|
