All about CPM and PERT (These are my notes from class, so this post will be “raw” without much thought for grammar and punctuation.)
My oversimplification of this Lesson 03. (Computation of project times is in the lesson as well. Again, slides are courtesy of Professor Rolly Torio.)
CPM – Critical Path Method
- Used for production management, where repetitive tasks can be determined and there is no need for much estimation.
- Created by Du Pont for the chemical plants.
- Focuses on the trade-off between time and cost.
- Single estimates of derterministic activity times
- Used more for project management, especially for intangible things such as research/development, where a lot of estimation time is involved.
- Developed by US Navy for the Polaris missile program.
- Focuses on getting the project done is as little time as possible.
- Multiple estimates of probabilistic activity times
Gantt Charts – provide easy graphical representations of when activiites will take place. BUT no details and no relationships between activities.
A Gantt Charts weaknesses may be address using
CPM + PERT (which overtime became one technique), the network technique.
-Useful for large projects
Network technique – shows the interdependence of activities through arrows.
Nodes (circle): EVENTS, where activities start or end, a task
Arrows: ACTIVITIES, effort required to perform work, completion of a task
All about the Times
Earliest Time – calendar time where an activity is completed, where all of its predecessor events are completed at their earliest times (best case).
Latest Time – latest calendar time an activity can occur without delaying subsequent events.
Slack Time = Latest START time – Earliest START Time
Positive Slack – amount of time an event can be delayed without delaying entire project
- sequence of events where there is NO slack
- longest path through a network
- minimum project completion time
Benefits of CPM/PERT:
- useful in all stages
- mathematically simple
- gives time: path and slack
- project documentation
- monitors cost
- clearly defined/independent activities
- specified precedence of relationships
- too much emphasis on critical paths