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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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### Transcript

• 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
• 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 -