Firstly I would like to introduce the two methods that use the network diagrams to give us a critical path in project management/planning.
1) Critical path method (CPM)
2) Program evaluation and review technique (PERT)
The network diagrams are constructed by generally following the steps in project plan. These steps can be considered as:
1) Breaking the tasks of a project down into a Work Breakdown Structure (WBS), which involves breaking down the project into “its major subcomponents (or tasks)” (Heizer & Render, 2009), followed by breaking down the major components into sub-components and further into activities in which detailed task lists can be defined.
2) Drawing up a Gantt Chart which represents the tasks on a chart that represents the time each task will take along a timeline, also showing overlaps where tasks may be done simultaneously. This chart will give a good overview of the timeline of activities to complete the project (Heizer & Render, 2009). The Gantt chart is actually one of the 3 alternative options after step 1, but if time permits it may be useful to prepare a Gantt chart as well. The PERT and CPM charts have an advantage over the Gantt charts as they offer a view of the relationships between activities and resources (Heizer & Render, 2009).
3) The second of the 3 options mentioned in point (2) is to draw up a CPM network. CPM was developed in 1957 by DuPont to address the challenges in shutting down and restarting chemical plants (NetMBA, n.d.). CPM represents tasks in a project as different nodes in a network and links them together based on the beginning and ending of a task (linking the ending of one task to the beginning of another) (NetMBA, n.d.).
4) The third of the 3 options is the PERT chart. It was developed in the late 1950s for the U.S Navy (NetMBA, n.d.) and tackles the major downfall of the CPM. The PERT chart follows the same steps of the CPM above and is represented in the same graphical method (a network); but takes into consideration the effect of time variations on a projects tasks (NetMBA, n.d.).
The major difference between PERT and CPM is that CPM gives each task (node) only one time estimate and PERT gives each task (node) 3 different time estimates when drawing up the network diagram (optimistic time, most likely time, pessimistic time).
To construct these charts there are 6 main steps involved (NetMBA, 2009):
1) Identify activities and milestones (i.e. point 1 above)
2) Identify the proper sequence of each activity (which follows which, etc. could use a Gantt chart of this).
3) Construct the network diagram.
4) Mark the time estimates for each activity on the network nodes of the diagram
5) Determine the critical path
6) Update the charts as the project progresses..
The critical path can be explained as determining the longest activity path in the project diagram by adding together the amount of time it takes to complete the tasks which rely on each other’s start and finish dates (NetMBA, n.d.). The critical path measures the full calendar length of a project from start to finish, if tasks outside of the critical path change their timelines (within limits) it should not affect the critical path (NetMBA, n.d). By identifying the critical path we can also identify the amount of time that other paths on the project network can be delayed by without affecting the timeline of the project, this is called the “slack time” (NetMBA, n.d.).
I think it is safe to say that the reason why it’s called the “critical” path is because of the fact that that specific path defines the length that the project is going to take, therefore it is the most important path to monitor.
Heizer, J. & Render, B. (2009) Operations Management. Ninth Edition. Prentice Hall: New Jersey.
NetMBA (n.d.) CPM – Critical Path Method [Online]. Available from: http://www.netmba.com/operations/project/cpm/ (Accessed: 14 May 2011).
NetMBA (n.d.) PERT [Online]. Available from: http://www.netmba.com/operations/project/pert/ (Accessed: 14 May 2011).