Program evaluation and review technique


The program 'evaluation and review technique' is a statistical tool used in project management, which was designed to analyze and represent the tasks involved in completing a given project.
PERT was originally developed by Charles E. Clark for the United States Navy in 1958; it is commonly used in conjunction with the Critical Path Method, which was also introduced in 1958.

Overview

PERT is a method of analyzing the tasks involved in completing a project, especially the time needed to complete each task, and to identify the minimum time needed to complete the total project. It incorporates uncertainty by making it possible to schedule a project while not knowing precisely the details and durations of all the activities. It is more event-oriented than start- and completion-oriented, and is used more for projects where time is the major constraint rather than cost. It is applied to very large-scale, one-time, complex, non-routine infrastructure projects, as well as R&D projects.
PERT offers a management tool, which relies "on arrow and node diagrams of activities and events: arrows represent the activities or work necessary to reach the events or nodes that indicate each completed phase of the total project."
PERT and CPM are complementary tools, because "CPM employs one time estimation and one cost estimation for each activity; PERT may utilize three time estimates and no costs for each activity. Although these are distinct differences, the term PERT is applied increasingly to all critical path scheduling."

History

PERT was developed primarily to simplify the planning and scheduling of large and complex projects. It was developed by the United States Navy Special Projects Office, Lockheed Aircraft, and Booz Allen Hamilton to support the Navy's Polaris missile project. It found applications throughout industry. An early example is the 1968 Winter Olympics in Grenoble which used PERT from 1965 until the opening of the 1968 Games. This project model was the first of its kind, a revival for the scientific management of Frederick Taylor and later refined by Henry Ford. DuPont's CPM was invented at roughly the same time as PERT.
Initially PERT stood for Program Evaluation Research Task, but by 1959 was renamed. It had been made public in 1958 in two publications of the U.S. Department of the Navy, entitled Program Evaluation Research Task, Summary Report, Phase 1. and Phase 2. both primarily written by Charles F. Clark. In a 1959 article in The American Statistician, Willard Fazar, Head of the Program Evaluation Branch, Special Projects Office, U.S. Navy, gave a detailed description of the main concepts of PERT. He explained:
Ten years after the introduction of PERT, the American librarian Maribeth Brennan compiled a selected bibliography with about 150 publications on PERT and CPM, all published between 1958 and 1968.
For the subdivision of work units in PERT another tool was developed: the Work Breakdown Structure. The Work Breakdown Structure provides "a framework for complete networking, the Work Breakdown Structure was formally introduced as the first item of analysis in carrying out basic PERT/CPM."

Terminology

Events and activities

In a PERT diagram, the main building block is the event, with connections to its known predecessor events and successor events.
  • PERT event: a point that marks the start or completion of one or more activities. It consumes no time and uses no resources. When it marks the completion of one or more activities, it is not "reached" until all of the activities leading to that event have been completed.
  • predecessor event: an event that immediately precedes some other event without any other events intervening. An event can have multiple predecessor events and can be the predecessor of multiple events.
  • successor event: an event that immediately follows some other event without any other intervening events. An event can have multiple successor events and can be the successor of multiple events.
Besides events, PERT also tracks activities and sub-activities:
  • PERT activity: the actual performance of a task which consumes time and requires resources. It can be understood as representing the time, effort, and resources required to move from one event to another. A PERT activity cannot be performed until the predecessor event has occurred.
  • PERT sub-activity: a PERT activity can be further decomposed into a set of sub-activities. For example, activity A1 can be decomposed into A1.1, A1.2 and A1.3. Sub-activities have all the properties of activities; in particular, a sub-activity has predecessor or successor events just like an activity. A sub-activity can be decomposed again into finer-grained sub-activities.

    Time

PERT defines four types of time required to accomplish an activity:
  • optimistic time: the minimum possible time required to accomplish an activity or a path, assuming everything proceeds better than is normally expected
  • pessimistic time: the maximum possible time required to accomplish an activity or a path, assuming everything goes wrong.
  • most likely time: the best estimate of the time required to accomplish an activity or a path, assuming everything proceeds as normal.
  • expected time: the best estimate of the time required to accomplish an activity or a path, accounting for the fact that things don't always proceed as normal.
  • standard deviation of time : the variability of the time for accomplishing an activity or a path

    Management tools

PERT supplies a number of tools for management with determination of concepts, such as:
  • float or slack is a measure of the excess time and resources available to complete a task. It is the amount of time that a project task can be delayed without causing a delay in any subsequent tasks or the whole project. Positive slack would indicate ahead of schedule; negative slack would indicate behind schedule; and zero slack would indicate on schedule.
  • 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. An activity with zero free float is not necessarily on the critical path since its path may not be the longest.
  • 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 reaches completion.
  • lag time: the earliest time by which a successor event can follow a specific PERT event.
  • fast tracking: performing more critical activities in parallel
  • crashing critical path: Shortening duration of critical activities

    Implementation

The first step for scheduling the project is to determine the tasks that the project requires and the order in which they must be completed. The order may be easy to record for some tasks while difficult for others. Additionally, the time estimates usually reflect the normal, non-rushed time. Many times, the time required to execute the task can be reduced for an additional cost or a reduction in the quality.

Example

In the following example there are seven tasks, labeled A through G. Some tasks can be done concurrently while others cannot be done until their predecessor task is complete. Additionally, each task has three time estimates: the optimistic time estimate, the most likely or normal time estimate, and the pessimistic time estimate. The expected time is computed using the formula ÷ 6.
Once this step is complete, one can draw a Gantt chart or a network diagram.

Next step, creating network diagram by hand or by using diagram software

A network diagram can be created by hand or by using diagram software. There are two types of network diagrams, activity on arrow and activity on node. Activity on node diagrams are generally easier to create and interpret. To create an AON diagram, it is recommended to start with a node named start. This "activity" has a duration of zero. Then you draw each activity that does not have a predecessor activity and connect them with an arrow from start to each node. Next, since both c and d list a as a predecessor activity, their nodes are drawn with arrows coming from a. Activity e is listed with b and c as predecessor activities, so node e is drawn with arrows coming from both b and c, signifying that e cannot begin until both b and c have been completed. Activity f has d as a predecessor activity, so an arrow is drawn connecting the activities. Likewise, an arrow is drawn from e to g. Since there are no activities that come after f or g, it is recommended to connect them to a node labeled finish.
File:pert example network diagram.gif|frame|center|A network diagram created using Microsoft Project. Note the critical path is in red.
By itself, the network diagram pictured above does not give much more information than a Gantt chart; however, it can be expanded to display more information. The most common information shown is:
  1. The activity name
  2. The expected duration time
  3. The early start time
  4. The early finish time
  5. The late start time
  6. The late finish time
  7. The slack
In order to determine this information it is assumed that the activities and normal duration times are given. The first step is to determine the ES and EF. The ES is defined as the maximum EF of all predecessor activities, unless the activity in question is the first activity, for which the ES is zero. The EF is the ES plus the task duration.
  • The ES for start is zero since it is the first activity. Since the duration is zero, the EF is also zero. This EF is used as the ES for a and b.
  • The ES for a is zero. The duration is added to the ES to get an EF of four. This EF is used as the ES for c and d.
  • The ES for b is zero. The duration is added to the ES to get an EF of 5.33.
  • The ES for c is four. The duration is added to the ES to get an EF of 9.17.
  • The ES for d is four. The duration is added to the ES to get an EF of 10.33. This EF is used as the ES for f.
  • The ES for e is the greatest EF of its predecessor activities. Since b has an EF of 5.33 and c has an EF of 9.17, the ES of e is 9.17. The duration is added to the ES to get an EF of 14.34. This EF is used as the ES for g.
  • The ES for f is 10.33. The duration is added to the ES to get an EF of 14.83.
  • The ES for g is 14.34. The duration is added to the ES to get an EF of 19.51.
  • The ES for finish is the greatest EF of its predecessor activities. Since f has an EF of 14.83 and g has an EF of 19.51, the ES of finish is 19.51. Finish is a milestone, so the EF is also 19.51.
Barring any unforeseen events, the project should take 19.51 work days to complete. The next step is to determine the late start and late finish of each activity. This will eventually show if there are activities that have slack. The LF is defined as the minimum LS of all successor activities, unless the activity is the last activity, for which the LF equals the EF. The LS is the LF minus the task duration.
  • The LF for finish is equal to the EF since it is the last activity in the project. Since the duration is zero, the LS is also 19.51 work days. This will be used as the LF for f and g.
  • The LF for g is 19.51 work days. The duration is subtracted from the LF to get an LS of 14.34 work days. This will be used as the LF for e.
  • The LF for f is 19.51 work days. The duration is subtracted from the LF to get an LS of 15.01 work days. This will be used as the LF for d.
  • The LF for e is 14.34 work days. The duration is subtracted from the LF to get an LS of 9.17 work days. This will be used as the LF for b and c.
  • The LF for d is 15.01 work days. The duration is subtracted from the LF to get an LS of 8.68 work days.
  • The LF for c is 9.17 work days. The duration is subtracted from the LF to get an LS of 4 work days.
  • The LF for b is 9.17 work days. The duration is subtracted from the LF to get an LS of 3.84 work days.
  • The LF for a is the minimum LS of its successor activities. Since c has an LS of 4 work days and d has an LS of 8.68 work days, the LF for a is 4 work days. The duration is subtracted from the LF to get an LS of 0 work days.
  • The LF for start is the minimum LS of its successor activities. Since a has an LS of 0 work days and b has an LS of 3.84 work days, the LS is 0 work days.