What is a MECE issue tree and how would you use one to structure a client problem?

MECE stands for Mutually Exclusive, Collectively Exhaustive. Applied to an issue tree, it means the branches at each level do not overlap and, together, cover the whole problem.

How to build one:

• Start from a precise question, not a topic. "Why has EBITDA fallen 12% year on year?" can be decomposed; "the business is underperforming" cannot.
• Choose a decomposition that is mathematically or logically complete. Profit splits into revenue and cost; revenue splits into price and volume; volume splits by product, channel or geography. Each split is MECE by construction, which is why these are used so heavily.
• Stop when a branch is directly testable with data you can actually obtain.
• Prune early. Size each branch roughly and drop the ones too small to matter. This is where the 80/20 principle does its work.

Note: The strongest version of this answer acknowledges that frameworks are a starting structure, not a checklist. Forcing a problem into a standard framework that does not fit is a common and visible mistake in case interviews.