Project Production Institute's recent web conference addressed the effectiveness of Critical Path Management (CPM) as a scheduling method for achieving project delivery success. From the history of CPM’s origination, to the mechanics of its application in practice, to experts’ experience applying CPM in practice, the evidence points to CPM as a theoretical framework that is not effective in application. There are other, more effective ways to plan and control project delivery.
The CPM concept is simply that the longest sequence of tasks, the critical path, will determine the duration of a project. Theoretically, that makes complete sense. The problem arises in execution. A project is a temporary endeavor undertaken to create a unique product, service, or result. Projects are made up of a variety of production systems. Production systems are sets of operations within a larger supply network or value chain that produces technical or physical output to satisfy an external demand. An easy way to think of this from a project perspective is that the project schedule provides demand for the project and the project production systems provide supply. A simple example is provided in Figure 1.
Figure 1: Production Required for Burgers Before the Game
The object for this project is for me and my wife to have a few guests over for burgers and salad, served outside in a Covid safe practices setting, before the game starts on TV this weekend. A CPM analysis shows that making and cooking the burgers is the critical path in this project with a duration of 148 minutes (90 + 20 + 20 + 3 + 5 + 10). The other items don’t take as long so have some “float” as to when they can be started and still finish on-time.
A schedule is generated from the project requirements and it generates demand on each of the capacity resources (me, my wife, the grill, and kitchen counter space for preparing the meal). Inventory is required for the project, including burgers, buns, and salad ingredients. Any project can be broken into five levers for planning and control: Product Design, Process Design, Capacity, Inventory and Variability. For more explanation see (Fischer & Massih, 2020).
Without going into detail on all the five levers, note that the CPM analysis does not explicitly take into account capacity resources. The assumption is that there is enough capacity to complete all the tasks and have the burgers and salad ready in time to eat before the game. That’s a pretty obvious problem and requires planning how many resources are needed to complete the project on-time. Looks like we will have to recruit an adult child or a guest to be bartender as there is too much time required for two people to do everything. If one person is working the 43 minutes required on burger production (no labor capacity is required for resting the burgers), there’s another 82 minutes of work required for other production activities and one person can’t do them all in parallel. Discrete capacity requirements have been recognized as a shortcoming of CPM and have never been fully addressed. However, that is not the only shortcoming of CPM.
Projects and their production systems are subject to variability in both supply and demand. Because of variability, the critical path will change from period to period. Deterministic schedules that define a critical path are out of date as soon as they are published.
Of course it never happens, but my wife and I might get in an argument causing us to leave the production lines for 15 minutes or so to cool down. The dog could knock over the salad bowl. The grill may run out of propane requiring an extra trip to the store. When unplanned events happen, we are not going to readjust our schedule and figure out the new critical path. We’re too busy trying to recover the meal production operation to get ready for the game. A critical path schedule cannot effectively be updated as quickly as changes occur.
Next time you walk into a project manager’s office, check the date of the project schedule being used. Chances are good that it will be weeks or months out of date. Even if the CPM schedule could be updated every day, there is no way the resources (trades, engineers, planners, etc.) of a typical project with its immense complexity would be able to respond to the changes effectively. Project production systems end up being run by the resources involved in each production system such as pipe spool ISO design and checking, pipe laying, rebar cage fabrication and installation, or precast fabrication and placement.
A more effective approach is to consider and manage the underlying behavior of a project’s production systems and the five levers for planning and control. Project production system behavior never lies about project work completion and holds the key to delivering projects on schedule and on budget. A CPM schedule, or any schedule, will provide information about what a project should do in terms of cost and duration. Schedules provide information necessary for any project. CPM schedules will not provide effective prediction or control about what can or will happen on projects.