The Robust Prerequisite Tree
By H. William Dettmer
A Brief History of the Thinking Process
The thinking process created by Eliyahu M. Goldratt has “morphed” over the past two decades. It was originally an integrated series of logic trees intended to analyze organizational systems with the objective of uncovering and breaking hidden systemic constraints. (Figure 1) Applying the thinking process of the early 1990s began with a Current Reality Tree to find the underlying policy—referred to as a “core problem”—that was limiting the ability of the organization to achieve its goal.
Once this core problem was identified, it was presumed that its existence stemmed from an underlying, hidden conflict, which the second step in the thinking process, the evaporating cloud, was specifically designed to resolve. The output of the evaporating cloud was an injection, so named by Goldratt because it was something that did not currently exist but which had to be “injected” in the situation in order to resolve the conflict perpetuating the limited organizational performance.
That injection then became the basis for the future reality tree, a chain of cause-and-effect that did not yet exist, but which, if put into place, would reverse the adverse effects of the core problem. Once the efficacy of the evaporating cloud injection was validated, and all other required supporting injections delineated, prerequisite trees were used to identify all obstacles to executing these injections and to brainstorm ways to overcome those injections. (Figure 2) Goldratt called these intermediate objectives. The obstacles and their paired intermediate objectives were arranged in sequence (and in parallel, where possible) for resolution.
But because the prerequisite tree did no more than identify obstacles and ways to overcome them, some means of actually executing all required actions was necessary, including but not limited to those called out in the prerequisite tree. For this, Goldratt proposed the transition tree. This tree was basically a kind of future reality tree reduced to the working level, a kind of work-process instructions for grass-roots execution. It was nearly three years after the thinking process was introduced in 1992 before the idea of decomposing the integrated thinking process into its individual tools and applying them separately was conceived.
A Problem with the Original Prerequisite Tree Concept
For more than 20 years, the Prerequisite Tree has been an important part of the thinking process, a significant element in execution of a solution. But there were problems associated with the prerequisite tree. Perhaps the most aggravating of these was the fact that it only addressed obstacles to implementation.
Certainly, it’s important to identify all the obstacles to a new course of action and to develop ways to overcome them. But that’s not all there is to execution. As with any complicated activity, there are challenges we must overcome, but there are many more routine tasks and actions that, though important in their own right, don’t have any obstacles to their completion. Yet we can’t afford to forget or overlook them. Goldratt intended for this additional “housekeeping” to be taken care of in the transition tree.
In the first ten years of teaching the thinking process to working professionals in all walks of life, I discovered a natural reticence on their part to confine the prerequisite tree to identifying obstacles and intermediate objectives to overcome them alone. In almost all cases, people included all the detailed tasks supporting execution of an injection along with the actions to overcome obstacles. And naturally, after doing so constructing the transition tree seemed superfluous and unnecessary.
For a few years, I tried to get my thinking process clients to avoid rolling everything into a prerequisite tree, without much success. Finally, I gave up and said, “Okay, put all the intermediate objectives—execution tasks—you believe you need to implement the injection, even those without associated obstacles. Just don’t contrive obstacles that don’t exist merely to adhere to a preconceived ‘paired’ format.” (Figure 3)
The Solution: A “Robust” Prerequisite Tree
The predisposition of my thinking process students to merge the functions of the prerequisite and transition trees forced me to re-think the way the prerequisite tree was constructed and, subsequently, a new way to use it in execution of injections. The result of this re-thinking was a much more robust tool. Implementing an injection directly from a “robust” prerequisite tree turned out to be an effective, efficient way to do it. (See Figure 4) Moreover, it was a lot easier for people to learn and even to apply as a stand-alone tool. But this facility is based on some underlying assumptions.
Completeness. The first assumption about the new, robust prerequisite tree is that it is, first and foremost, a complete accounting of all the intermediate objectives—necessary conditions—required for effective, efficient execution of a particular injection. Some of these intermediate objectives are needed to overcome obstacles, which are also articulated in the prerequisite tree. But considerably more are merely important tasks or accomplishments that must not be overlooked.
Project-oriented. The second underlying assumption is that execution of an injection may not be complicated enough to require a prerequisite tree. But when it is complicated, especially when an implemented injection represents the outcome of a set of interdependent activities, a prerequisite tree can start to take on overtones of a project: many of its component activities are sequentially dependent. Some sequences can be accomplished in parallel, but all sequences ultimately converge at the top of the tree in “project delivery”— implementation of the injection.
System knowledge. The third underlying assumption is that the people who will be constructing complicated thinking process analyses are primarily working professionals, even if they apply the logic trees to a topic outside their professional work environments. In other words, they already have enough intuitive knowledge of their systems and how they operate without the need for the pedantic, step-by-step work task instructions that were the chief stock-in-trade of the transition tree.
“Projectizing” the Prerequisite Tree
One of the early tasks in planning a project is developing a work breakdown structure (WBS). Once the WBS is completed, the relationship and dependencies among component activities can be charted. Typically, this picture of the project is embodied in a project activity network that forms the basis for a PERT (program evaluation review technique) chart. (Figure 5) If you’re inclined to manage injection implementation as a project, the prerequisite tree can help you depict the sequence and parallelism in implementation activities.
Moreover, it’s very easy to integrate other elements of project management with the prerequisite tree:
Accountability. An office, or officer, of primary responsibility (OPR) can be assigned to be accountable for the successful implementation of the prerequisite tree. This is crucial for unity of command and assignment of responsibility.
Performance. Clearly defined measures of success for each intermediate objective can be established, so that all can know when the task is complete.
Resources. Each intermediate objective in the prerequisite tree can be matched with the resources (human, tools, equipment, information, and financial) required to successfully complete it.
Schedule. PERT diagrams typically include estimates of task duration. While not as comprehensive as a formal PERT chart, the time to complete the intermediate objectives in a prerequisite tree can be estimated and included.
Figure 6 shows how this “projectizing” of injection execution might look.
Adapted from Dettmer, H. William, The Logical Thinking Process: A Systems Approach to Complex Problem Solving, Milwaukee, WI: ASQ Quality Press, 2007, Ch.7.
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