Critical chain scheduling

Critical chain scheduling is a method of improving schedules. Critical chain schedules were discussed in Chapter 5. In a critical chain schedule, buffers are created between the early schedule completion date of the project and the promise date. The activities that have float are first moved to their late schedule dates and then moved back the amount of buffer that they are given from the point where they join the critical path.

One of the important things that critical chain schedules do is to recognize that by delaying the feeder chains toward the late schedule, the risks are reduced. Risks are reduced in the later schedule because knowledge learned in the course of doing the project can be applied to these activities because they have been delayed from their early schedule. Early in the project most of the project team is inexperienced, and mistakes are much more likely to be made. Later in the project experience has been gained by all, which means that risks that might otherwise cause trouble can be avoided.

An important part of the risk quantification process is the ranking of the risks into the order of severity. When quantitative or semiquantitative analysis is done, this is relatively simple. The expected value of the risk can be used to list the risks. Using the expected value, the risks with the highest expected value are going to be at the top of the list. This is simply because the risks with the highest expected value are the ones that, on average, will cost the project the most.

Even if we are not using expected value analysis to evaluate the risks, we can still rank them. If we are using semi-quantitative techniques, we can multiply the values that we used to qualitatively estimate the impacts and probabilities to get a relative severity. This will be fine as long as the relative scale used for evaluating probability and impact is the same for all of the risks.

In the case of purely qualitative evaluations, judgment can be used to rank the risks. There are several methods for doing this. One of the easiest is to have each person in a meeting rank the risks individually and then combine the rankings of each person into a composite ranking. This will give an overall consensus ranking of the risks.

Another method that can be used either on an individual basis or by groups is the comparison matrix.

In the comparison matrix each risk is numbered from 1 to the highest number of the risks. Numbers assigned to the risks have no particular meaning. In Figure 8-10 the risks are numbered 1 through 7. Above the risk numbers are the comparison boxes. These are organized to ensure that two risks at a time are compared and that every risk is compared to every other risk. Human brains have trouble dealing with seven things simultaneously, but they can easily deal with two things. The comparison matrix allows many items to be ranked by comparing only two items at a time.

In the column above risk 1, the comparisons made, starting at the top of the column, are: 1 to 2, 1 to 3, 1 to 4, 1 to 5, 1 to 6, and 1 to 7. In subsequent columns the numbers of comparisons are smaller since there is no point in repeating comparisons: 1 compared to 2 is the same as 2 compared to 1. In comparing and ranking risks, each time a comparison is made, the more severe risk is noted in the comparison of only those two risks. Each time a risk is considered to be the more severe one, a hash mark is put in the box below that risk. After all comparisons have been made, the risk with the highest number of hash marks is the most severe risk. The one with the next highest number of marks is ranked number two, and so on.

Comparison matrix

Figure 8-10: Comparison matrix

 
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