Critical Path Method (CPM)
Critical Path Method (CPM)
By Umesh Dwivedi
The Critical Path Method (CPM) is a schedule network analysis technique. CPM was developed by the DuPont Corporation in 1957. The critical path determines the shortest time to complete the project and it is the longest duration path through a network of tasks. Critical tasks (activities) are tasks (activities) on the critical path.
To understand CPM further let’s first understand nature of the task. According to PMBOK every scheduled task can be defined by the following four parameters.
- Early Start (ES): Earliest possible point in time on which a task can start.
- Early Finish (EF): Earliest possible point in time on which a task can finish.
- Late Start (ES): Latest possible point in time on which a task can start.
- Late Finish (EF): Latest possible point in time on which a task can finish.
Early Start and finish dates are calculated by means of Forward Pass and Late Start and Late Finish dates are calculated by means of Backward Pass.
Many Tasks have some amount of buffer added to them referred as Slack Time or Float. Float time is the amount of time a task can slip before it delays the project schedule. There are two common types of floats:
- Free Float: Amount of time a single task can be delayed without delaying the early start of any successor task.
- Total Float: Amount of time a single task can be delayed without delaying project completion.
Mathematically Float is defined as: Float = LS – ES or LF – EF.
The critical path has zero or negative Total Float. A project can have several critical paths.
- Project Planning and control.
- Time-cost trade-offs.
- Cost-benefit analysis.
- Contingency planning.
- Reducing risk.
Limitations of CPM:
- CPM assumes low uncertainty in schedule dates.
- Does not consider resource dependencies.
- Less efficient use of buffer time.
- Less focus on non critical tasks that can cause risk.
- Based on only deterministic task duration.
- Critical Path can change during execution.
To overcome the above limitations the Critical Chain Project Management (CCPM) plays important role.
Umesh Dwivedi is certified PMP (Project Management Professional) from PMI, USA. He is experienced in project planning, execution, monitoring and controlling of projects. He has worked in collaboration with cross-functional, cross-organizational teams for many clients internationally.
Visit his home page at http://www.freewebs.com/umeshspace.
I think there is a small mistake in the Critical Path calculation: task1 is 6 days long and not 5 days, so the total path should be 22 and not 21 days.
It seems a typo, so everything else looks correct.
best regards,
Riccardo Orfei, PMP
I have a little question. can we have two critical paths in one project?
Ricardo,
I have fixed the mistake in the critical path calculation, the problem was that task1 was displayed as 5 days instead of 6, the calculation, however, is still correct at 21 days.
Shafeeq, you can have two (or more) critical paths in the same project when they have the exact same length. However, as the project progresses, you will discover that only one of them is valid.
Sorry guys, but after being a planner / scheduler for over 15 years I can confidently say that this is a flawed definition of the CPM scheduling. For example, the critical path can not only have zero or negative total float, but can also have positive total float. Additionally, try defending a lag on your critical path in claims litigation – if there’s an event, add an activity for it (especially if it’s a critical path activity!!!). If not, then the succeeding activity should start sooner. Finally, I find it amusing that the examples given are illustrated in Microsoft Project, which is NOT a CPM scheduling tool.