We’ve talked a few times about being mutually exclusive and collectively exhaustive (or MECE) in your thinking:
First, we talked about how MECE thinking is useful because it helps ensure that your approach has no overlaps (ME) and no gaps (CE). Then we looked at ways to be more MECE in your thinking by being CEME. And we’ve also addressed the fundamental issue of MECE thinking in problem solving: that your true intent is not to find solutions that are truly mutually exclusive but rather independent because being ME requires a preclusion. So, rather, we introduced the idea that you should think about being ICE (independent and collectively exhaustive) instead. This seems to be confusing, so let’s see if we can clarify the whole thing.
In short, make your thinking MECE and your answers ICE. Let me explain.
Your issue tree should have MECE branches. Consider the poster child of issue trees, the profitability tree, below.
Forcing the branches to be truly ME is good because it prevents redundancies: if you have included an idea in, say, the “revenues” branch, then it shouldn’t appear in the “costs” one. However, the answers (that is, the elements in the tree) are not ME: maybe you have a profitability problem because your revenues are too low and your costs are too high. Having one problem does not preclude you from also having the other. These elements are independent (i.e., one does not require the help of another to be an answer to your stem question).
And this is fine, this is what you want: an efficient map (a tree without redundancies) leading to a set of elements several of which might be an answer to your question.
For some problems, making your answers be independent will also force them to be mutually exclusive. In the example below, assuming that you are traveling only once from NYC to London and that whichever means of transportation you choose takes you the entire way, choosing a plane, say, precludes you from choosing any other means of transportation.
But this is a characteristic of the problem rather than a property of your solution, so you should not worry about it.
In the end, the branches / organization is MECE. The solutions are ICE and sometimes MECE.