Critical Path Method
Limitations of the Critical Path Method deal with critical tasks, fixed duration and scheduled dates. The Critical Path Method has been widely used in project planning. The Critical Path Method or CPM, was developed in 1957 as a joint venture between the DuPont chemical company and Remington Rand Univac computer firm.
In fact, two mathematicians, James Kelly and Morgan Walker, devised CPM during their attempt to reduce the costs of plant shutdowns and restarts which occurred due to inefficient scheduling.
The Critical Path Method is useful in project planning and control, cost-benefit analysis, risk reduction and contingency planning But there are limitations to the CPM. If you know the limitations correctly, then you can look out for signs of method failure.
Limitations of Critical Path Method – Critical Tasks
First and foremost, CPM focuses only on critical tasks. In fact, we cannot categorize tasks as critical and non-critical because non-critical tasks can become critical and critical tasks can become non-critical at any time.
You might not have focused on a task considering it as non-critical. But, during the project execution, that particular non-critical task might become critical and introduce some kind of risk.
While depicting a project in the CPM network, the whole project is broken down into small activities or tasks and the CPM does not help you in tracking resources independently of project activities.
The network assigns a specific duration or schedule to each activity which has no prerequisites. The duration is not well known for new types of projects. So, it is difficult to plan new projects using this method.
Limitations of Critical Path Method – Fixed Duration
Also, there are certain activities that cannot be practically assigned fixed duration. For example, if you consider a drawing activity, it is difficult to assign a fixed duration because the whole activity is continuous.
So such activities are difficult to accommodate in the CPM network. Moreover, it is difficult to estimate the duration of activities in complex, multidimensional projects. If you cannot estimate duration accurately, then critical path identification becomes a guessing game. The Critical Path Method does not use buffer time efficiently.
Another item is that CPM assumes low uncertainty in schedule dates. The fact is that we cannot expect that all tasks will be completed exactly as per the schedule. Unexpected issues like change of requirements, unavailability of employees etc might delay the schedule.
Even with estimates of lower accuracy, the CPM works if the deviations balance out. In other words, CPM works even if some tasks take longer and some take shorter. But, in real projects, other tasks will have problems if a task takes longer than scheduled. So, if any activity on the critical path takes longer than scheduled, then the whole project will have to suffer.
The Critical Path Method, as the name indicates, focuses on critical path in project planning. But the fact is that critical path can change during execution. When there is some problem in a project, usually project managers change tasks and create new paths to address the problem.
The CPM network is complicated even for small projects. In multidimensional and complex projects, the CPM design becomes unmanageable complicated and time consuming. Most of these limitations are overcome with Critical Chain Project Management (CCPM).