Critical Path Method

In Chapter 8 we mentioned that CPM is similar to PERT. In the original versions of CPM and PERT there was one important difference: CPM included a way of relating the project schedule to the level of physical resources allocated to the project. This allowed the PM to trade time for cost, or vice versa. In CPM, two activity times and two costs are specified, if appropriate, for each activity. The first time/cost combination is called normal and the second set is referred to as crash. Normal times are "normal" in the same sense as the m time estimate of the three times used in PERT. Crash times result from an attempt to expedite the activity by the application of additional resources—for example, overtime, special equipment, additional staff or material, and the like.

It is standard practice with PERT/CPM to estimate activity times under the assumption of resource loadings that are normal. To discuss a time requirement for any task without some assumption about the level of resources devoted to the task makes no real sense. At the same time, it does not make sense to insist on a full list of each and every resource that will be spent on each of the hundreds of activities that may comprise a PERT/CPM network. Clearly, there must have been some prior decision about what resources would be devoted to each task, but much of the decision making is, in practice, relegated to the common methods of standard practice and rules of thumb. The allocation problem requires more careful consideration if it is decided to speed up the accomplishment of tasks and/or the total project. We need to know what additional resources it will take to shorten completion times for the various activities making up the project.

While standard practice and rules of thumb are sufficient for estimating the resource needs for norma! progress, careful planning is critical when attempting to expedite (crash) a project. Crash plans that appear feasible when considered activity by activity may incorporate impossible assumptions about resource availability. For example, we may need to crash some activities on the Wild Horse Dam Project. To do so, we have all the labor and materials required, but we will need a tractor-driven crawler crane on the project site not later than the eighth of next month. Unfortunately, our crane will be in Decatur, Illinois, on that date. No local contractor has a suitable crane for hire. Can we hire one in Decatur or Springfield and bring ours here?

And so it goes. When we expedite a project, we tend to create problems; and the solution to one problem often creates several more problems that require solutions. ■

Difficulties notwithstanding, the wise PM adopts the Scout's motto: "Be pre-, pared." If deterministic time estimates are used, and if project deadlines are firm,, there is a high likelihood that it will be necessary to crash the last few activities of most projects. Use of the three probabilistic time estimates of PERT may reduce the chance that crashing will be needed because they include uncertainties that are sometimes forgotten or ignored when making deterministic time estimates. Even so, many things make crashing a way of life on some projects—things such as last-minute changes in client specifications, without permission to extend the project deadline by an appropriate increment. An example of one of the problems that commonly result from the use of deterministic time estimates can be seen in the boxed example that follows.

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Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

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