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Project Scheduling: PERT/CPM

Project Scheduling: PERT/CPM. Characteristics of a Project. A unique, one-time effort Requires the completion of a large number of interrelated activities Resources, such as time and/or money, are limited Typically has its own management structure. Project Management.

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Project Scheduling: PERT/CPM

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  1. Project Scheduling: PERT/CPM M. En C. Eduardo Bustos Farias

  2. Characteristics of a Project • A unique, one-time effort • Requires the completion of a large number of interrelated activities • Resources, such as time and/or money, are limited • Typically has its own management structure M. En C. Eduardo Bustos Farias

  3. Project Management • A project manager is appointed to head the project management team • The team members are drawn from various departments and are temporarily assigned to the project • The team is responsible for the planning, scheduling and controlling the project to its completion M. En C. Eduardo Bustos Farias

  4. PERT and CPM • PERT: Program Evaluation and Review Technique • CPM: Critical Path Method • Graphically displays project activities • Estimates how long the project will take • Indicates most critical activities • Show where delays will not affect project M. En C. Eduardo Bustos Farias

  5. M. En C. Eduardo Bustos Farias

  6. Project Schedule • Converts action plan into operating timetable • Basis for monitoring & controlling project activity • More important for projects than for day-to-day operations • projects lack continuity of on-going functions • more complex coordination needed • One schedule for each major task level in WBS • Maintain consistency among schedules • Final schedule reflects interdependencies, departments. M. En C. Eduardo Bustos Farias

  7. Network Model • Serves as a framework for: • planning, scheduling, monitoring, controlling • interdependencies and task coordination • when individuals need to be available • communication among departments and functions needed on the project • Identifies critical activities and slack time • Reduces interpersonal conflict M. En C. Eduardo Bustos Farias

  8. PERT / CPM • PERT: • Program Evaluation and Review Technique • estimates probability of on-time completion • CPM: • Critical Path Method • deterministic time estimates • control both time and cost • Similar purposes, techniques, notation • Both identify critical path and slack time • Time vs. performance improvement M. En C. Eduardo Bustos Farias

  9. PERT / CPM Definitions • Activity: task or set of tasks • uses resources and takes time • Event: result of completing an activity: • has identifiable end state at a point in time • Network: combined activities & events in a project • Path: series of connected activities • Critical: activities, events, or paths which, if delayed, will delay project completion • Critical path: sequence of critical activities from start to finish • Node / Arrow (Arc) - PERT / CPM notation M. En C. Eduardo Bustos Farias

  10. The Basics of Using PERT/CPM M. En C. Eduardo Bustos Farias

  11. The Project Network Model M. En C. Eduardo Bustos Farias

  12. PERT / CPM Notations • EOT: • earliest occurrence time for event • time required for longest path leading to event • LOT: latest occurrence time for event • EST: earliest starting time for activity • LST: latest starting time for activity • Critical time: shortest time in which the project can be completed • Notation: AOA, AON, dummy activities M. En C. Eduardo Bustos Farias

  13. Slack Time M. En C. Eduardo Bustos Farias

  14. Project Network M. En C. Eduardo Bustos Farias

  15. Example M. En C. Eduardo Bustos Farias

  16. Partial Network How should activity K be added? M. En C. Eduardo Bustos Farias

  17. This works, but there is a better way. M. En C. Eduardo Bustos Farias

  18. M. En C. Eduardo Bustos Farias

  19. Earliest Time for an Event M. En C. Eduardo Bustos Farias

  20. Earliest Time for Each Event Expected time to complete the project is 44 days. M. En C. Eduardo Bustos Farias

  21. Latest Time for an Event M. En C. Eduardo Bustos Farias

  22. Latest Time for Each Event Expected time to complete the project is 44 days. M. En C. Eduardo Bustos Farias

  23. Slack Time M. En C. Eduardo Bustos Farias

  24. Critical Activities M. En C. Eduardo Bustos Farias

  25. Probabilistic Time Estimation M. En C. Eduardo Bustos Farias

  26. Expected completion time: • Based on optimistic, pessimistic, most likely • Take weighted average of the 3 times • TE = (a + 4m + b)/6 • Uncertainty = variance (range of values) • Probability of completion of project in desired time D M. En C. Eduardo Bustos Farias

  27. Transforming Plan to Network M. En C. Eduardo Bustos Farias

  28. Know activities which comprise project • Determine predecessor and successor activities • Time and resources for activities • Interconnections depend on technical interdependencies • Expected completion time • as soon as possible versus as late as possible M. En C. Eduardo Bustos Farias

  29. GANTT Chart M. En C. Eduardo Bustos Farias

  30. Gantt Charts Henry Laurence Gantt (1861-1919) M. En C. Eduardo Bustos Farias

  31. Planned and actual progress • for multiple tasks on horizontal time scale • easy to read, easy to construct • effective monitoring and control of progress • requires frequent updating M. En C. Eduardo Bustos Farias

  32. Components of GANTT Chart • Activities - scheduled and actual • Precedence relationships • Milestones (identifiable points in project) • usually represents reporting requirements • usually corresponds to critical events • Can add budget information • Does not show technical interdependencies • Need PERT network to interpret, control, and compensate for delays M. En C. Eduardo Bustos Farias

  33. Planning and Scheduling M. En C. Eduardo Bustos Farias

  34. Gantt Basics • Basically, a timeline with tasks that can be connected to each other • Note the spelling! • It is not all-capitals! • Can be created with simple tools like Excel, but specialised tools like Microsoft Project make life easier M. En C. Eduardo Bustos Farias

  35. Making a Gantt chart • Step 1 – list the tasks in the project M. En C. Eduardo Bustos Farias

  36. Making a Gantt chart • Step 2 – add task durations M. En C. Eduardo Bustos Farias

  37. Making a Gantt chart • Step 3 – add dependencies (which tasks cannot start before another task finishes) M. En C. Eduardo Bustos Farias

  38. Notes • The arrows indicate dependencies. • Task 1 is a predecessor of task 2 – i.e. task 2 cannot start before task 1 ends. • Task 3 is dependent on task 2. Task 7 is dependent on two other tasks • Electrics, plumbing and landscaping are concurrent tasks and can happen at the same time, so they overlap on the chart. All 3 can start after task 4 ends. • Painting must wait for both electrics and plumbing to be finished. • Task 9 has zero duration, and is a milestone M. En C. Eduardo Bustos Farias

  39. Making a Gantt chart • Step 4 – find the critical path The critical path is the sequence of tasks from beginning to end that takes the longest time to complete. It is also the shortest possible time that the project can be finished in. Any task on the critical path is called a critical task. No critical task can have its duration changed without affecting the end date of the project. M. En C. Eduardo Bustos Farias

  40. MS Project can work out the critical path for you! • The length of the critical path is the sum of the lengths of all critical tasks (the red tasks 1,2,3,4,5,7) which is 2+3+1+1.5+2+1 = 10.5 days. • In other words, the minimum amount of time required to get all tasks completed is 10.5 days • The other tasks (6,8) can each run over-time before affecting the end date of the project M. En C. Eduardo Bustos Farias

  41. The amount of time a task can be extended before it affects other tasks is called slack (or float). • Both tasks 6 and 8 can take one extra day before they affects a following task, so each has one day’s slack. M. En C. Eduardo Bustos Farias

  42. Critical tasks, by definition, can have NO slack. Tip: If ever asked Can task X’s duration be changed without affecting the end date of the project?, if it is a critical task the answer is always NO! M. En C. Eduardo Bustos Farias

  43. Benefits of CPM/PERT • Useful at many stages of project management • Mathematically simple • Give critical path and slack time • Provide project documentation • Useful in monitoring costs M. En C. Eduardo Bustos Farias

  44. Advantages of PERT/CPM • useful at several stages of project management • straightforward in concept, and not mathematically complex • uses graphical displays employing networks to help user perceive relationships among project activities • critical path and slack time analyses help pinpoint activities that need to be closely watched • networks generated provide valuable project documentation and graphically point out who is responsible for various project activities • applicable to a wide variety of projects and industries • useful in monitoring not only schedules, but costs as well M. En C. Eduardo Bustos Farias

  45. Limitations to CPM/PERT • Clearly defined, independent and stable activities • Specified precedence relationships • Subjective time estimates • Over emphasis on critical paths M. En C. Eduardo Bustos Farias

  46. Limitations of PERT/CPM • project activities must be clearly defined, independent, and stable in their relationships • precedence relationships must be specified and networked together • time activities in PERT are assumed to follow the beta probability distribution -- this may be difficult to verify • time estimates tend to be subjective, and are subject to fudging by managers • there is inherent danger in too much emphasis being placed on the critical path M. En C. Eduardo Bustos Farias

  47. Probabilistic PERT/CPM M. En C. Eduardo Bustos Farias

  48. Mean and Standard Deviation of Project Duration • Once the expected time te for all activities has been computed, proceed to use te in place of the single activity duration in CPM to work out the critical path and the project duration • The resulting project duration is the mean project duration TE • We also need to work out the standard deviation of the project duration  as follows: • Project Duration = (Summation of i2 f all the activities on the critical path) M. En C. Eduardo Bustos Farias

  49. Probability of Different Project Durations • From statistics, once we know the mean project duration, TE, and the standard deviation of the project duration,  we can work out the probability that the project duration will be shorter than any specific time, T (i.e. the project will take T days or less) through the following formula: • Z=(T- TE )/  , where Z is the quantity called the Normal variate • Knowing Z, we can read off the probability from Normal Distribution Tables which are provided in nest slides M. En C. Eduardo Bustos Farias

  50. Normal Distribution Table for Negative Values of Z M. En C. Eduardo Bustos Farias

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