Is CPM Wrong or is it Just That Some People Think Wrongly about CPM?

I was going back through some backlogged reading and re-read Hal Macomber's post about CPM and I had to respond.

In it he says the following:

Schedules are constructed using estimates of task durations and task dependencies. Goldratt showed us the effects of dependence and variation on the predictability or reliability throughout a project. At best we have reasonable estimates. But they are estimates. Rarely do people take the time to produce estimated ranges of effort and duration. Consequently, our schedules fail to represent the stochastic (probabilistic) nature of the project. Yet we treat our schedules as if they are deterministic. This is the fundamental flaw in our use of the critical path method.

I have seen this kind of comment lots of times from people that use TOC or XP or Agile, etc methods for their PM and they all seem to imply by these comments that the CPM method itself is flawed. But is it really? The CPM method as I was thought it simply says that based on the dependencies within a set of tasks that there are some of the tasks that will likely have a greater impact on the schedule if they slip. I don't remember it being said that these where the ONLY tasks to be watched.

On the issue of estimates I could not agree more with Hal when he says that estimates are just that ESTIMATES. But the problem with their lack of quality is not the fault of the CPM method they are the fault of sloppy estimating. And along the same lines I also agree that PMs that see their schedules as deterministic are flawed in their thinking. I mentioned this same idea in an article I wrote for Techrepublic called New MS Project users often run from reality. But again are we talking about a failure of CPM to be a valuable tool for project managers or the failure of project managers to use the tool? I think this is an important distinction that often gets lost in the fray of "New Method" vs. "Old Method" discussions.

To be certain I am not fully versed in the ways of TOC, Agile, XP, etc. I am working hard to get up to speed on these topics as we speak, so I'm no expert. However, I think that saying that CPM is flawed BECAUSE some of it's practitioners are wrong in their use is throwing the baby out with the bathwater.

Comments

[Frank Winters]

"Just an estimate?" Yep, that's what it is until you base commitments on it. If you are estimating so you can tell powerful people when you will have what done, you better triangulate (use three differing estimation methods and then rationalize them) and use tools like CPM to help you understand your own data. Saying there is no critical path is like saying there is no hammer.

[Jack Dahlgren]

Brian,
I think you are very right about this. Most experienced CPM practitioners understand that the schedule is just an estimate and that it is one possible model. TOC or any other methodology (after all - TOC is just CPM with contingency anyway) do not change this.