You make some good points on the use of Monte Carlo, etc. … My comment was based on what I have observed in the software industry. While I do not have the experience to completely back up this claim, I expect that large scale construction projects such as the ones to which you refer suffer from different kinds of chronic problems than software development. In my experience, focus on CPM has more to do with ensuring that the PM escapes blame than with ensuring that the project is completed on schedule, on budget and (most importantly) on target with respect to user/customer expectations.

I think you need to look around at aerospace and large construction - both my area of experience to see of “a cock” holds there. I just came from a $4B manned spaceflight program architect role where probabilistic CPM along with the deterministic CPM are the core program management tools.

TOC is used in this environment as well, but TOC in aerospace depends on discovering where the schedule needs to be put. This starts with building the deterministic schedule, then analyzing the probabilistic completion dates and assigning buffer in front of the appropriate points. The critical issues here is to assign buffers whose unused margin can be moved to the future. It does not good to have unused margin that can not “move to the left” future deliverables - other than to protect that one activity.

Probabilistic CPM is the tool used to this approach - both Risk+ and @Risk for Project are standards in aerospace.

Using CPM for the right reasons, adding the Monte Carlo simulation and assigning proper protection margin is the core of any aircraft, weapons systems or space flight project for the past 15 years. “A Crock” probably means lack of understanding of how these pr ojects are managed on-time, on-budget, on-spec.

There is an excellent intro to Theory of Constraints at http://www.dbrmfg.co.nz/Preface.htm that I think establishes a basis for a huge improvement over CPM. If anyone is interested, I am also working on a series of essays on project management. The first (Why Projects Fail) is posted on my rather eclectic blog … http://hither-and-yon.blogspot.com/2006/11/why-projects-fail.html

This is the same old song. Project management teams that are too lasy to keep up their schedules. Ill prepared to actually “manage” the project in a proactive way.

In the government contracting domain as well as large construction at places like CH2M Hill it is simply not allowed to not know the probabilistic. critical path on a week by week basis.

David Schmaltz refers to a paper about the issues with the CPM method. The referred paper is really about the PERT estimating parameters. These problems have been well documented in the literature for decades - which is likely why the paper did not get accepted at Cross Talk. See “PERT Completion Times Revisited,” Ted Williams, University of Michigan-Flint as a starting point for a survey of the source as well as the solution to the problem.

Monte Carlo simulations with risk adjusted task completion estimates derived in a variety of ways is the standard approach on NASA and Defense systems Integrated Master Plan / Integrated Master Schedule programs prescribed by DID 81650.

My personal experience includes Crew Exploration Vehicle ($3.8B), Hubble Robotic Servicing Mission ($670M) as well as working directly with those having managed 100’s of Billions of dollars of other spacecraft and launch vehicle projects - all on time, budget, on specification. The I-25 TREX project just completed under budget and 8 months ahead of schedule here in Denver using probabilistic critical path management techniques.

It is not the tool that is failing the project. It is the failure of the people managing the project to adequately understand and mitigation the inherent uncertainties that reside in the project. Start with “Quantitative Risk Analysis for Project Management: A Critical Review,” Lionel Galway, RAND Working Paper, WR-112-RC, Feb 2004.

For the treatise on probabilistic risk management for projects, see Effective Risk Management: Some Key to Success, 2nd Edition, Edmund H. Conrow, AIAA Press, 2003. The materials in this book are mandatory for any aerospace or defense system program manager.

A final note on the “wrongness” of the CPM calculation. This is well known and well understood in many domains. The solution is provided in several tools - Risk+, @Risk for Project, SAS, and Crystal Ball. Restating the problem without a known solution adds little to the profession of Project Management.

Any wishing to learn more about the probabilistic management of complex projects, drop me a note for the introduction briefing we provide to all entry level planners in the spacecraft development business

Glen B. Alleman
Practice Director
Strategy and Performance Management
galleman@lewisandfowler.com

I’d be interested to see what this service delivers, too. I have available a paper, written by a real-live scientist and project practitioner, which shows that the assumptions behind CPM, the actual formula used to calculate Critical Path, is wrong. Anyone interested, send me a note (david@projectcommunity.com), and I’ll forward a copy of “A Treatise On Project Time Estimation” by Mark G. Gray (Los Alamos National Laboratory scientist).

Your acknowledgment that the promise of CPM is rarely delivered in practice is further encouragement to use the technique as a high-level guideline, and to not become entranced with its alluring promise. Even if the math worked (it doesn’t), knowing for certain what will happen next has never inoculated anyone against what actually happens next. The problem is, was, and ever will be the certainty such presumptions project.

david