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Project Duration Estimating Techniques: CPM vs. PERT

Explore methods for estimating project duration including CPM and PERT, critical path analysis, optimizing plans, and defense strategies. Learn the key concepts and techniques used in project management to ensure efficient and successful project completion.

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Project Duration Estimating Techniques: CPM vs. PERT

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  1. ELC 347 DAY 8

  2. Agenda • Questions • Assignment 3 Corrected • 4 A’s , One non-submit • Assignment #4 Due • Capstone Proposal Due • Group Progress report due • Exam 2 • Slight Change • 10 M/C (5 points each) 5 essays (10 points each) • 1 hour 15 minutes • Open book, Open Notes • Ghattas 4-8, Project 8-10 • Group Project Meeting (30 Min) • Discussion on Network Analysis and Duration Estimating

  3. Network Analysis and Duration Estimating • Laying the Groundwork for Commitment • Setting Duration Estimates • Determining the critical Path • Meeting external deadlines

  4. Groundwork for commitment • Ask the people that actually doing the work • If more than one is capable ask the one with the most experience • Be realistic • Be an advocate for your team

  5. Duration Estimating • Duration – the elapsed time from the start of an activity until it is finished • Effort -- the actual time spent on the project • Example • Tony work on a project task for 20 hours at 4 hours per day starting Monday mourning. The project task was completed Late Friday • Effort – 20 hours • Duration – 5 days

  6. Experience Historical data Research Modeling Experiments Breakdown/roll-up Delphi Method Panel of experts Consultants Three outside Estimates Ranging Other techniques Techniques for Estimating Duration

  7. Special Cases (Out of your hands) • Inspections • Deliveries • Breakdowns and repairs • Approvals

  8. Normalizing Duration • Goal is most “cost-efficient” plan • Determine the most cost-efficient technical approach • Make and estimate of long the most cost efficient method will take to complete • Selectively adjust you estimate for any activity that is subject to common problems.

  9. CPM versus PERT • PERT was developed by US Navy in the 1950s’ • CPM was developed by Remington Rand and DuPont around the same time. • Only difference is in durations estimating • Pert uses 3 cases Most optimistic, most pessimistic and most likely and determines probability for each • DoPo + DpOp +DePe = final duration • CPM use only the most likely duration

  10. Calculating a Project Duration • Determine each activity and its predecessors • Determine an estimated duration for each activity • Find the “Critical Path” • Add up the durations along the critical path

  11. Whats the “Critical Path” • The longest path based on precedence of activities and durations through a PERT/CPM network • It’s critical because • Its combined length determines the length of the project • It has NO slack • Delay of any activities on the critical path delays the entire project

  12. Finding the critical path • Use “forward pass” Calculations • Each activity should have • EPS earliest possible Start time • EPF  earliest possible finish time • DUR  duration • EPF = EPS + DUR

  13. Example

  14. Special Case • If an activity has more than one processor its EPS is set to the latest EPF of all its processors

  15. An example of forward pass

  16. Backwards Pass • Use to calculate Slack • LAS -> Latest start time • LAF -> Latest finish time • TS -> total slack

  17. An example of backward pass

  18. Examine Critical Path for Reality Check • Is the total duration typical? • Are the durations of the CP activities typical? • Reexamine all CP activity durations • Reexamine assumptions

  19. Optimizing the plan • Would bringing more expert talent to bear on the project speed it up? • Are there changes we could make in the resource procurement process to shorten wait times? • Would round the clock scheduling shorten duration? • Would more money motivate the team to work faster? • Would an organizational change speed things up? • Do we think it will take this long just because it always has, or are there other ways to so this, which we haven’t thought of yet, that would shrink the timetable?

  20. 3 Lines of defense • Relieve the critical path • Sharpen your pencil • Crash activities

  21. Summary • Construct a CPM • Estimate normal durations • Do a forward pass • Identify Critical Path • Optional • Do a backward pass to identify slack times • Conduct a reality check • Optimize the plan • 1st line of defense - relieve the critical path • 2nd line of defense - sharpen your pencil • 3rd line of defense - crash activities

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