An issue tree, also called "logic tree" or "issue map", is a graphical breakdown of a question that dissects it into its different components vertically and that progresses into details as it reads to the right.
Issue trees are useful in problem solving to identify the root causes of a problem as well as to identify its potential solutions. They also provide a reference point to see how each piece fits into the whole picture of a problem.
There are two types of issue trees: diagnostic ones and solution ones.
Diagnostic trees breakdown a "why" key question, identifying all the possible root causes for the problem. Solution tree breakdown a "how" key question, identifying all the possible alternatives to fix the problem.
To be effective, an issue tree needs to obey four basic rules:
- Consistently answer a “why” or a “how” question
- Progress from the key question to the analysis as it moves to the right
- Have branches that are mutually exclusive and collectively exhaustive (MECE)
- Use an insightful breakdown
The requirement for issue trees to be collectively exhaustive implies that divergent thinking is a critical skill.
A profitability tree is an example of an issue tree. It looks at different ways in which a company can increase its profitability. Starting from the key question on the right, it breaks it down between revenues and costs, and break these down into further details.
Having identified the different sources of risk for an engineering system and or
activity and analysed these in respect to their chronological and causal
components logical trees may be formulated and used for the further analysis of
the overall risk as well as the risk contribution of the individual components.
In the present chapter we will consider the basic aspects of some of the most
commonly used types of logical trees, namely fault trees, event trees, cause consequence
charts and decision trees.
Fault trees and event are by far and large the most well-known and most widely
applied type of logical tree in both qualitative and quantitative risk analysis. Two
of the most important risk studies involving fault tree and event tree analysis were
the US nuclear safety study and the UK Canvey study of chemical process
industries. Even though more modern risk analysis techniques such as e.g.
Bayesian Probabilistic Nets have been developing over the last years fault trees
and event trees are still the main methods recommended for US nuclear safety
Fault trees and event trees are in many ways similar and the choice of using one
or the other or a combination of both in reality depends more on the traditions
and preferences within a given industry than the specific characteristics of the
A significant difference between the two types of trees is though that whereas the
fault trees take basis in deductive (looking backwards) logic the event trees are
inductive (looking forward). In practical applications a combination of fault trees
and event trees is typically used where the fault tree part of the analysis in
concerned about the representation of the sequences of failures, which may lead
to events with consequences and the event tree part of the analysis which is
concerned with the representation of the subsequent evolution of the
consequence inducing events.
The intersection between the fault tree and the event tree is in reality a matter of
preference of the engineer performing the study. Small event tree / large fault
tree and large event tree / small fault tree techniques may be applied to the same
problem to supplement each other and provide additional insight in the
performance of the considered system. Cause consequence charts incorporate significant features of fault and event
trees and are in principal just a combination of the two.
Decision trees are often seen as a special type of event tree, but may in fact bee
seen in a much wider perspective and in fact applied consistently within the
framework of decision theory provides the theoretical basis for risk analysis.
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