Issue trees, logic trees, “how” trees, “why” trees, decision trees, fact trees, hypothesis trees… Your favorite consultancy has its own terminology and so does the rest of the world. Unfortunately, they don’t coincide, so here is an attempt at making things clearer.

The short answer is that there isn’t a clear nomenclature that everyone uses. At Accenture, we called both “why” and “how” trees “issue trees”. That’s because both types raise and answer questions or issues—we used the two terms interchangeably. McKinsey talk about “logic trees” in the same way.

Then you’ll hear people talk about hypothesis trees and fact trees or data trees. The first ones are useful to uncover the root causes of a problem and the other two help identify potential solutions. But that, in my opinion, is misleading: indeed, both hypotheses and facts are present in all types of trees, so calling a tree by the type of elements it contains doesn’t really help. Instead, I think it makes more sense to call them by what they do.

So here is what I think makes the most sense:

1. There are two types of trees: “why” and “how”. A “why” tree is a diagnostic tree: its objective is to identify the root causes of the problem. A “how” tree is (almost) a decision tree: its objective is to identify various ways in which one can solve the problem. (A “how” tree is not entirely a decision tree because, these, usually, end up with the expected value of each branch; that is not a common feature in a “how” tree.)

2. Both types of trees are logic trees or issue trees. The aim of both types of trees is to bring logic in the problem, helping us be MECE, so it makes sense to call them logic trees. But both types of trees are also question- (or issue)-based, so it makes sense to call them issue trees as well.

3. Both trees raise questions and include hypotheses. Furthermore, both trees spell out the analysis needed to test these hypotheses and uncover facts. When asking “how do I get from NYC to London?”—which is a “how” tree so, one would think, a fact/data-tree—I need to make hypotheses. By plane, by boat, by submarine, swimming, etc. are all possible solutions and, indeed, must be included in the tree but until I have validated that they are indeed potential acceptable answers to my original problem they remain hypotheses.

4. You can’t mix “why” and “how” questions in one same tree. Because a tree’s functionality is either to understand the root causes of the problem or to identify potential ways to solve it but not both at the same time, you can’t have both types of questions in the same structure. I am not saying that you should answer only why or how questions in your analysis; indeed, in general, a complete analysis requires to first develop a “why” tree and then develop a “how” tree. I am saying that both these types of questions are important, but they don’t come at the same time in the analysis. So, first, you understand the real problem (e.g. do I have a headache because I’m dehydrated, because I have brain cancer, because of another reason) then you solve it (e.g. I’ll take an Advil vs. I’ll start chemotherapy).