Project Management

PERT

last edited by: Thiago Iglesias on Mar 27, 2015 3:07 PM login/register to edit this page

Contents
1 Instructions
2 Benefits
3 References

PERT (Program Evaluation Review Technique) is a planning and control tool used for defining and controlling the tasks necessary to complete a project. PERT charts and Critical Path Method (CPM) charts are often used interchangeably; the only difference is how task times are computed. Both charts display the total project with all scheduled tasks shown in sequence. The displayed tasks show which ones are in parallel, those tasks that can be performed at the same time. A graphic representation called a "Project Network" or "CPM Diagram" is used to portray graphically the interrelationships of the elements of a project and to show the order in which the activities must be performed.

Instructions

PERT planning involves the following steps:

1. Identify the specific activities and milestones. The activities are the tasks of the project. The milestones are the events that mark the beginning and the end of one or more activities.

2. Determine the proper sequence of activities. This step may be combined with step 1 above since the activity sequence is evident for some tasks. Other tasks may require some analysis to determine the exact order in which they should be performed.

3. Construct a network diagram. Using the activity sequence information, a network diagram can be drawn showing the sequence of the successive and parallel activities. Arrowed lines represent the activities and circles or "bubbles" represent milestones.

4. Estimate the time required for each activity. Weeks are a commonly used unit of time for activity completion, but any consistent unit of time can be used. A distinguishing feature of PERT is it's ability to deal with uncertainty in activity completion times. For each activity, the model usually includes three time estimates:

  • Optimistic time - the shortest time in which the activity can be completed.
  • Most likely time - the completion time having the highest probability.
  • Pessimistic time - the longest time that an activity may take.
From this, the expected time for each activity can be calculated using the following weighted average:

Expected Time = (Optimistic + 4 x Most Likely + Pessimistic) / 6

This helps to bias time estimates away from the unrealistically short timescales normally assumed.

5. Determine the critical path. The critical path is determined by adding the times for the activities in each sequence and determining the longest path in the project. The critical path determines the total calendar time required for the project. The amount of time that a non-critical path activity can be delayed without delaying the project is referred to as slack time. If the critical path is not immediately obvious, it may be helpful to determine the following four times for each activity:

  • ES - Earliest Start time
  • EF - Earliest Finish time
  • LS - Latest Start time
  • LF - Latest Finish time
These times are calculated using the expected time for the relevant activities. The earliest start and finish times of each activity are determined by working forward through the network and determining the earliest time at which an activity can start and finish considering its predecessor activities. The latest start and finish times are the latest times that an activity can start and finish without delaying the project. LS and LF are found by working backward through the network. The difference in the latest and earliest finish of each activity is that activity's slack. The critical path then is the path through the network in which none of the activities have slack.

The variance in the project completion time can be calculated by summing the variances in the completion times of the activities in the critical path. Given this variance, one can calculate the probability that the project will be completed by a certain date assuming a normal probability distribution for the critical path. The normal distribution assumption holds if the number of activities in the path is large enough for the central limit theorem to be applied.

6. Update the PERT chart as the project progresses. As the project unfolds, the estimated times can be replaced with actual times. In cases where there are delays, additional resources may be needed to stay on schedule and the PERT chart may be modified to reflect the new situation.

Benefits

Benefits to using a PERT chart or the Critical Path Method include:

  • Improved planning and scheduling of activities.
  • Improved forecasting of resource requirements.
  • Identification of repetitive planning patterns which can be followed in other projects, thus simplifying the planning process.
  • Ability to see and thus reschedule activities to reflect interproject dependencies and resource limitations following know priority rules.
  • It also provides the following: expected project completion time, probability of completion before a specified date, the critical path activities that impact completion time, the activities that have slack time and that can lend resources to critical path activities, and activity start and end dates.

References

  1. Project Management: Tools & Techniques. Sean Maserang. MSIS 488: Systems Analysis & Design. Fall 2002. http://www.umsl.edu/~sauterv/analysis/488_f02_papers/ProjMgmt.html
  2. The Numbers Group. "Project Management". http://www.snc.edu/socsci/chair/333/numbers.html. Last update time unknown. Accessed Nov. 4, 2002.
  3. "What is Project Management?". http://www.esi_intl.com/Public/projectmanagement/whypm.asp. Last update time unknown. Accessed Nov. 4, 2002.
  4. Georgia State University - CIS 330. "Project Management". http://www.cis.gsu.edu/~dtruex/courses/cis330/cis330PDF/wk7/ProjMgtp.pdf.
  5. NetMBA. "PERT". http://www.netmba.com/operations/project/pert.


last edited by: Thiago Iglesias on Mar 27, 2015 3:07 PM login/register to edit this page


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