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Program Evaluation and Review Technique

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A discussion on the basics of creating a PERT chart …

A discussion on the basics of creating a PERT chart

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  • 1. Program Evaluation Review Technique (PERT) Report by: Raymund N. Sanchez
  • 2. Content of the Presentation
    • Definition
    • Differences between PERT & CPM
    • Purpose
    • Historical Perspective
    • Terminologies
    • Creating a PERT/CPM diagram
    • Schedule Duration Crash
    • Probabilistic Time Estimates
      • uncertainty of activities and paths
      • path probabilities
    • Problem Exercises
  • 3. Definition
    • A method to analyze the tasks involved in completing a given project.
    • Focus is paid to the time needed to complete each task, and identifying the minimum time needed to complete the total project.
  • 4. Purpose
    • To simplify the planning and scheduling of large and complex projects.
    • To incorporate uncertainty in the sense that it was possible to schedule a project not knowing precisely the details and duration's of all the activities.
    • Event-oriented technique rather than start- and completion-oriented.
    • Used more in R&D-type projects where Cost is not a major factor but Time is.
  • 5. PERT & CPM Similarities
    • Both follow the same steps and use network diagrams
    • Both are used to plan the scheduling of individual activities that make up a project
    • They can be used to determine the earliest/latest start and finish times for each activity
  • 6. PERT & CPM Differences
    • PERT is probabilistic whereas CPM is deterministic
    • In CPM, estimates of activity duration are based on historical data
    • In PERT, estimates are uncertain and we talk of ranges of duration and the probability that an activity duration will fall into that range
    • CPM concentrates on Time/Cost trade off.
  • 7. Historical Background
    • PERT was invented by Booz Allen Hamilton, Inc. under contract to the United States Department of Defense's US Navy Special Projects Office in 1958
    • A part of the Polaris mobile submarine-launched ballistic missile project. This project was a direct response to the Sputnik crisis
    • CPM was developed by the dupont company & Remington-Rand-Univac
    • Used for large construction projects
    • Each were unaware of the others existence until the 1960’s
  • 8. Terminologies
    • 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. It is not “reached” until all of the activities leading to that event have been completed.
    • P redecessor 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.
    • 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.
  • 9. Terminologies
    • PERT activity : is the actual performance of a task. It consumes time , it requires resources (such as labor, 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).
  • 10. Terminologies
    • Most likely time (M): the best estimate of the time required to accomplish a task, assuming everything proceeds as normal.
    • Expected time (T E ): 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).
    • 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.
  • 11. Terminologies
    • 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 .
  • 12. Terminologies
    • Early Start (ES): maximum EF of all predecessor activities, unless the activity in question is the the first activity, wherein ES is 0
    • Early Finish (EF): ES plus task duration
    • Late Start (LS): LF minus task duration
    • Late Finish (LF): minimum LS on all successor activities, unless the activity is the last activity, wherein LF equals EF
    • Activity on Arrow (AOA): a type of PERT diagram wherein the activities are written on the arrows
    • Activity on Node (AON): a type of PERT diagram wherein the activities are written on the nodes
  • 13. Creating a PERT Diagram
    • STEPS 1:
    • Determine the tasks that the project requires and the order in which they must be completed
    • Determine the optimistic, most likely, and pessimistic time of each task
    • Compute for the Expected time using the formula
    • Te=(O+4M+P)/6
    • Determine whether to use AOA or AON diagrams
  • 14.  
  • 15. Start F C G E D B A Finish
  • 16. Creating a PERT Diagram
    • STEPS 2:
    • Determine the ES & EF of each activity by:
      • Start at the beginning moving towards the end
      • ES & EF for the start activity is always 0 since they are milestones
      • Use the EF of the predecessor activity as the ES of the current activity
      • EF of an activity is computed by adding its ES with its duration
      • For activities with 2 or more predecessor activities, use the predecessor with the higher EF as the ES of the current activity
  • 17. Start ES:0 EF:0 F D:4.5 ES:10.33 EF:14.83 C D:5.17 ES:4 EF:9.17 G D:5.17 ES:14.34 EF:19.51 E D:5.17 ES:9.17 EF:14.34 D D:6.33 ES:4 EF:10.33 B D:5.33 ES:0 EF:5.33 A D:4 ES:0 EF:4 Finish D:0 ES:19.51 EF:19.51
  • 18. Creating a PERT Diagram
    • STEPS 3:
    • Determine the LS & LF of each activity by:
      • Start at the end and work towards the beginning
      • The LF for the finish activity is equal to EF since it is the last activity in the project. Since duration is 0, LS is equal to LF
      • Use the LS of the successor activity as the LF of the current activity
      • LS of an activity is computed by subtracting its LF with its duration
      • For activities with 2 or more successor activities, use the successor with the lower LS as the LF of the current activity
  • 19. Start D:0 ES:0 EF:0 LS:0 LF:0 F D:4.5 ES:10.33 EF:14.83 LS:15.01 LF:19.51 C D:5.17 ES:4 EF:9.17 LS:4 LF:9.17 G D:5.17 ES:14.34 EF:19.51 LS:14.34 LF:19.51 E D:5.17 ES:9.17 EF:14.34 LS:9.17 LF:14.34 D D:6.33 ES:4 EF:10.33 LS:8.68 LF:15.01 B D:5.33 ES:0 EF:5.33 LS:3.84 LF:9.17 A D:4 ES:0 EF:4 LS:0 LF:4 Finish D:0 ES:19.51 EF:19.51 LS:19.51 LF:19.51
  • 20. Creating a PERT Diagram
    • STEPS 4:
    • Compute for the critical path by adding the duration's of various paths for all activities
    • Determine if any activities have slack by subtracting the activity’s LF & EF
  • 21. Critical Path
    • Critical Path: A-C-E-G
    • Path A-D-F = 14.83 work days
    • Path A-C-E-G = 19.51 work days
    • Path B-E-G = 15.67 work days
  • 22. Slack
  • 23. Gantt Chart
  • 24. Schedule Duration Crash
    • Crash : an effort to reduce the overall time duration of a project by employing one or all of the following techniques
      • Adding resources (human or otherwise)
      • Increasing work hours (overtime or weekends)
      • Lessening quality
    • A trade-off between shorter task duration and higher task costs
    • If the cost savings on a delay penalty are higher than the incremental cost of reducing the project duration, then the crashing is justified.
  • 25. Activity Uncertainty
    • Standard Deviation of an activity is estimated as one sixth of the difference between the pessimistic and optimistic time estimates
    • Variance is determined by squaring the standard deviation
    • The size of the variance reflects the degree of uncertainty associated with the activity’s time. The larger the variance, the greater the uncertainty.
    • Standard Deviation = t p - t o
    • 6
  • 26. Path Uncertainty
    • Standard Deviation of a path can also be computed to know the uncertainty of a particular path.
    • SD of Path=  variances of activities on a path
  • 27. Path Probability
    • The probability that a given path will be completed in a specified length of time can be determined using the following formula:
    • Z = Specified Time - Path Mean
    • Path Standard Deviation
    • If the value of Z is 2.50 more, treat the path probability as 100%. If the value of Z is less than 2.50, use the table of values under the standardized normal curve.
  • 28. Sample Problem
  • 29. - END -

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