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RESOURCE AND BOTTLENECK BUFFERS 87

On the one hand, the problem (as described by Goldratt) is that the project
schedule contingency is usually added to invisible task contingency—resulting in
an unreasonably extended schedule, and relieving pressure to get jobs done in the
shortest reasonable time. On the other hand, one has to be careful not to be
overzealous in the reduction of contingency so as to put the project at risk for
meeting contract required dates.

The concept of removing the contingency from individual tasks and placing it
in project and feeder buffers makes a lot of sense and is highly recommended. It
is a simple idea that can be easily applied. Goldratt omits discussion of tools to aid
in the application of his methods. We will provide such advice later in this chap-
ter. Goldratt goes on to state three policies associated with the basic concept. I
have some serious reservations on these.

First, he suggests that we use remaining duration rather than percent complete
to measure task status. This makes sense, but only if you are not employing earned
value analysis techniques (which I almost always recommend). Remaining dura-
tion has always been a statusing option in most project management software.

He also states that “all we are concerned with is the critical path.” But then,
Goldratt goes on to contradict himself by discussing means of using feeder buffer
analysis to manage the schedule. I prefer using EVA (BCWP vs. BCWS) to ana-
lyze work production (as discussed further in this chapter, and in Section 8).
However, a formal application of Feeder Buffer analysis can be used as an alter-
native method.

I strongly disagree with Goldratt’s policy that we eliminate management by
milestones. Milestones are good for interim accomplishment (providing immedi-
ate recognition and reward). Some milestones are required interim targets. The
use of selected milestones should be retained to supplement other measure-
ments. I almost always develop a Project Milestone Schedule for initial top-down
planning and as a guide for detailed planning.

Resource and Bottleneck Buffers

Up to this point, Goldratt deals with a scheduling model, wherein potential re-
source conflicts and limits are not considered . He goes on to addresses the com-
plexity of resource constraints. He introduces resource buffers and directs that
the critical path now must go through all tasks that involve simultaneous con-
tention for scarce resources. He inserts resource buffers when work shifts to a
new resource. These resource buffers are applied only to the critical chain, placed
before critical tasks to alert resources of pending work. Goldratt acknowledges, at
this point, that computer support is needed for analysis and determination of re-
source overloads and scheduling.