Critical Path Method: Fool Me Once, Fool Me Twice
By Hal Macomber
Fool Me Once -- Task durations are estimates. The critical path method (CPM) is considered THE standard for managing projects. Customer contracts often require developing and maintaining the critical path schedule in great detail. Universities teach CPM in project management courses. CPM is the primary function of thebest-selling project management software. Large plots of project schedules hang in construction trailers and project management offices depicting the network diagram and the critical path. No project professional in his or her right mind would start a project without calculating the critical path. So, if it is so widely used, then why are projects late, over budget, and dissatisfying customers? We are fooledby the critical path. The central presumption for establishing a critical path is that we know how long each activity or task will take. When the activities are then strung together according to precedence relationships one can find the minimum time through the project. That is the “critical path.” So what is the problem? How could you know what the real time will be for completing a task? Youcan’t. It is complicated by not knowing exactly who will be performing the task. (Rookies take longer than experienced people.) And it is further complicated by not knowing the circumstances (or situation) for performing the task. (Even experienced people can be distracted or can have an “off” day.) So what does this mean for anyone managing projects? If you think that managing a project means justkeeping your eye on the critical, then you are mistaken. Knowing who will perform and the circumstances for performance make more of a difference. Don’t be fooled. Fool Me Twice -- Task durations are fabrications. Let’s say you produce a critical path (for whatever reason). The generally accepted approach is to ask We don’t know all of what must be done. Oftentimes ad hoc work (those tasks thatseem to arise in the course of doing the other work) each key performer to provide durations for the tasks and the precedence relationships. With this data you can find the longest path through the network of tasks. With this approach you overcome one of the problems previously identified. So, is there still a problem? You bet. Success with the critical path method hinges on knowing task durations –how else are we to coordinate action? Each person will estimate the time it will take them to perform. If they are at all risk averse, then they will also buffer that duration based on their experience performing similar tasks. Why? Because they don’t want to be the person responsible for getting the project off track. However, we don’t know what those buffers are. One person might add a 20%buffer while another adds a 500% buffer. Eli Goldratt, author of Critical Chain, and founder of the Avraham Goldratt Institute, suggests we can safely assume that all durations are at least twice as long as they need to be. What are we to do? We must investigate task level of effort (estimated hours to perform) for every task on the critical path and consider carefully its application. Durations aloneare not sufficient. We are fooled twice when we accept durations as stated. Fool Me Again -- Task durations vary. Experienced project managers will tell you the critical path moves on a project. Why? Tasks don’t start and finish as represented in the project schedule. This would be fine if all the performers for critical path tasks were always available to perform on the project, but this is notthe case. In most organizations people are working on more than one project at a time or project work is in addition to their normal work responsibilities. This creates the situation where they must manage priorities – “Do I spend my time on this or on that?”
©2002 Hal Macomber. From the weblog Reforming Project Management, http://weblog.halmacomber.com. Reprint is authorized with inclusion...
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