CPM: Fool Me Twice
October 1st, 2002 by HalArticle Series - CPM: Fool Me Once, 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 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.
We can safely assume that all durations are at least twice as long as they need to be.
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 alone are not sufficient. We are fooled twice when we accept durations as stated.
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October 6th, 2002 at 6:19 pm
Frank Patrick’s comments are consistent with the experience in using the Last Planner™ System. A while back the LPS was conceived to address the unreliablility in project execution. Without going into all of it in a ‘comment’ I’ll say that dependence and variability (as described by Goldratt) were the breakthrough ideas that led to the current incarnation of the LPS.
While there’s no arguing that some resources can be contraints on a project, all resources, specifically people who promise, can add to the variability experienced on the project. Attending to the reliability of the promising of all while giving extra attention to the constraining resources has resulted in significant performance improvements on projects.
BTW, you’ll never catch me arguing with Goldratt. The Theory of Constraints is an incredible contribution to production management, especially one-of-a-kind production, a.k.a. ‘projects’.