Limitations of the Critical Path Method

The critical path method, or CPM, is a staple of project management. You identify the tasks that have to be done to finish the project, the order in which they should be performed and how long that sequence will take. The result is the shortest possible time in which you can hope to complete the project. It's a useful tool as long as you understand the advantages and disadvantages of CPM.

Tip

Critical path analysis can simplify large projects by identifying the sequence of steps to complete them. The critical path method may become confusing if the project is too large, and it doesn't adapt well if you have to make revisions in the middle of the project.

When you apply CPM to a new project, you identify the tasks that have to be done and the sequence for completing them. If you have several different sequences, you map them out visually, showing each path proceeding from the project start to its finish. One of these paths will be the critical path.

Suppose your two tasks are to develop a new sweetener and bring it to the market. One path involves giving the product a name, trademarking the name and creating a marketing campaign. Another path involves the lab research to create the sweetener, test it for safety and test it for taste.

When you map these out, you see the two paths as two separate sequences. When you assign completion times to the tasks in each path, you realize that the lab work will take much longer. The most efficient sequence of the combined two paths is your critical path.

In any given project, some paths have slack, meaning you can postpone or delay them and it won't affect the deadline for finishing the project. In the critical path, that isn't the case. Any delay in the sequence of tasks means you miss your deadline.

For example, suppose your task is opening a new store. The critical path involves finding the right location, buying it and renovating it to suit your concept. You've calculated that this will take two years total with three months allotted to finding the location.

If the location hunt takes six months, everything else in the critical path gets pushed back. You can't buy the property let alone remodel it until the first step is complete. You'll have to revise your timeline.

The CPM gives you the absolute minimum time in which the project can be completed. You know which tasks have to be done in which order and the earliest points at which they can start. This makes CPM analysis a useful tool for setting timetables and figuring out the staffing and resources you'll need to complete the project.

Because the CPM lays out the paths visually, it helps you understand the parallel activities running alongside the critical path and helps keep track of them. You know which tasks are critical and those that aren't, which shows you which tasks to prioritize. In large projects with multiple paths and component tasks, CPM can save you from losing your focus.

One of the critical path analysis disadvantages, however, is that if the project gets too big, CPM isn't helpful. If you find yourself drawing so many paths that your chart looks like a spider web, CPM has probably maxed out its effectiveness. That's one of the problems with the critical path method.

Complexity is only one of the critical path analysis disadvantages.

• One of the problems with the critical path method is that it doesn't adapt well to making changes on the fly. If your project is stable and reasonably predictable, then CPM can be useful, but if you have to improvise a lot, then it's not very useful.
  
  
• Drawing a CPM diagram can take a lot of time and effort. In some projects, identifying which of the paths is critical may be difficult.
  
  
• You may have to guesstimate a lot of the project task durations, making the timetable less reliable and CPM less useful.
  
  
• Do you control the resources and staff you need to complete the critical path on deadline? If not, your schedule may be hopelessly optimistic.